| Message ID | 20220722071228.146690-7-ebiggers@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | make statx() return DIO alignment information | expand |
On 07/22, Eric Biggers wrote: > From: Eric Biggers <ebiggers@google.com> > > Currently, if an f2fs filesystem is mounted with the mode=lfs and > io_bits mount options, DIO reads are allowed but DIO writes are not. > Allowing DIO reads but not DIO writes is an unusual restriction, which > is likely to be surprising to applications, namely any application that > both reads and writes from a file (using O_DIRECT). This behavior is > also incompatible with the proposed STATX_DIOALIGN extension to statx. > Given this, let's drop the support for DIO reads in this configuration. IIRC, we allowed DIO reads since applications complained a lower performance. So, I'm afraid this change will make another confusion to users. Could you please apply the new bahavior only for STATX_DIOALIGN? > > Signed-off-by: Eric Biggers <ebiggers@google.com> > --- > fs/f2fs/file.c | 3 +-- > 1 file changed, 1 insertion(+), 2 deletions(-) > > diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c > index 5e5c97fccfb4ee..ad0212848a1ab9 100644 > --- a/fs/f2fs/file.c > +++ b/fs/f2fs/file.c > @@ -823,7 +823,6 @@ static inline bool f2fs_force_buffered_io(struct inode *inode, > struct kiocb *iocb, struct iov_iter *iter) > { > struct f2fs_sb_info *sbi = F2FS_I_SB(inode); > - int rw = iov_iter_rw(iter); > > if (!fscrypt_dio_supported(inode)) > return true; > @@ -841,7 +840,7 @@ static inline bool f2fs_force_buffered_io(struct inode *inode, > */ > if (f2fs_sb_has_blkzoned(sbi)) > return true; > - if (f2fs_lfs_mode(sbi) && (rw == WRITE)) { > + if (f2fs_lfs_mode(sbi)) { > if (block_unaligned_IO(inode, iocb, iter)) > return true; > if (F2FS_IO_ALIGNED(sbi)) > -- > 2.37.0
On Sat, Jul 23, 2022 at 07:01:59PM -0700, Jaegeuk Kim wrote: > On 07/22, Eric Biggers wrote: > > From: Eric Biggers <ebiggers@google.com> > > > > Currently, if an f2fs filesystem is mounted with the mode=lfs and > > io_bits mount options, DIO reads are allowed but DIO writes are not. > > Allowing DIO reads but not DIO writes is an unusual restriction, which > > is likely to be surprising to applications, namely any application that > > both reads and writes from a file (using O_DIRECT). This behavior is > > also incompatible with the proposed STATX_DIOALIGN extension to statx. > > Given this, let's drop the support for DIO reads in this configuration. > > IIRC, we allowed DIO reads since applications complained a lower performance. > So, I'm afraid this change will make another confusion to users. Could > you please apply the new bahavior only for STATX_DIOALIGN? > Well, the issue is that the proposed STATX_DIOALIGN fields cannot represent this weird case where DIO reads are allowed but not DIO writes. So the question is whether this case actually matters, in which case we should make STATX_DIOALIGN distinguish between DIO reads and DIO writes, or whether it's some odd edge case that doesn't really matter, in which case we could just fix it or make STATX_DIOALIGN report that DIO is unsupported. I was hoping that you had some insight here. What sort of applications want DIO reads but not DIO writes? Is this common at all? - Eric
On Jul 25, 2022, at 12:12 PM, Eric Biggers <ebiggers@kernel.org> wrote: > > On Sat, Jul 23, 2022 at 07:01:59PM -0700, Jaegeuk Kim wrote: >> On 07/22, Eric Biggers wrote: >>> From: Eric Biggers <ebiggers@google.com> >>> >>> Currently, if an f2fs filesystem is mounted with the mode=lfs and >>> io_bits mount options, DIO reads are allowed but DIO writes are not. >>> Allowing DIO reads but not DIO writes is an unusual restriction, which >>> is likely to be surprising to applications, namely any application that >>> both reads and writes from a file (using O_DIRECT). This behavior is >>> also incompatible with the proposed STATX_DIOALIGN extension to statx. >>> Given this, let's drop the support for DIO reads in this configuration. >> >> IIRC, we allowed DIO reads since applications complained a lower performance. >> So, I'm afraid this change will make another confusion to users. Could >> you please apply the new bahavior only for STATX_DIOALIGN? >> > > Well, the issue is that the proposed STATX_DIOALIGN fields cannot represent this > weird case where DIO reads are allowed but not DIO writes. So the question is > whether this case actually matters, in which case we should make STATX_DIOALIGN > distinguish between DIO reads and DIO writes, or whether it's some odd edge case > that doesn't really matter, in which case we could just fix it or make > STATX_DIOALIGN report that DIO is unsupported. I was hoping that you had some > insight here. What sort of applications want DIO reads but not DIO writes? > Is this common at all? I don't think this is f2fs related, but some backup applications I'm aware of are using DIO reads to avoid polluting the page cache when reading large numbers of files. They don't care about DIO writes, since that is usually slower than async writes due to the sync before returning from the syscall. Also, IMHO it doesn't make sense to remove useful functionality because the new STATX_DIOALIGN fields don't handle this. At worst the application will still get an error when trying a DIO write, but in most cases they will not use the brand new STATX call in the first place, and if this is documented then any application that starts to use it should be able to handle it. Cheers, Andreas
On 07/25, Eric Biggers wrote: > On Sat, Jul 23, 2022 at 07:01:59PM -0700, Jaegeuk Kim wrote: > > On 07/22, Eric Biggers wrote: > > > From: Eric Biggers <ebiggers@google.com> > > > > > > Currently, if an f2fs filesystem is mounted with the mode=lfs and > > > io_bits mount options, DIO reads are allowed but DIO writes are not. > > > Allowing DIO reads but not DIO writes is an unusual restriction, which > > > is likely to be surprising to applications, namely any application that > > > both reads and writes from a file (using O_DIRECT). This behavior is > > > also incompatible with the proposed STATX_DIOALIGN extension to statx. > > > Given this, let's drop the support for DIO reads in this configuration. > > > > IIRC, we allowed DIO reads since applications complained a lower performance. > > So, I'm afraid this change will make another confusion to users. Could > > you please apply the new bahavior only for STATX_DIOALIGN? > > > > Well, the issue is that the proposed STATX_DIOALIGN fields cannot represent this > weird case where DIO reads are allowed but not DIO writes. So the question is > whether this case actually matters, in which case we should make STATX_DIOALIGN > distinguish between DIO reads and DIO writes, or whether it's some odd edge case > that doesn't really matter, in which case we could just fix it or make > STATX_DIOALIGN report that DIO is unsupported. I was hoping that you had some > insight here. What sort of applications want DIO reads but not DIO writes? > Is this common at all? I think there's no specific application to use the LFS mode at this moment, but I'd like to allow DIO read for zoned device which will be used for Android devices. > > - Eric
On Sat, Jul 30, 2022 at 08:08:26PM -0700, Jaegeuk Kim wrote: > On 07/25, Eric Biggers wrote: > > On Sat, Jul 23, 2022 at 07:01:59PM -0700, Jaegeuk Kim wrote: > > > On 07/22, Eric Biggers wrote: > > > > From: Eric Biggers <ebiggers@google.com> > > > > > > > > Currently, if an f2fs filesystem is mounted with the mode=lfs and > > > > io_bits mount options, DIO reads are allowed but DIO writes are not. > > > > Allowing DIO reads but not DIO writes is an unusual restriction, which > > > > is likely to be surprising to applications, namely any application that > > > > both reads and writes from a file (using O_DIRECT). This behavior is > > > > also incompatible with the proposed STATX_DIOALIGN extension to statx. > > > > Given this, let's drop the support for DIO reads in this configuration. > > > > > > IIRC, we allowed DIO reads since applications complained a lower performance. > > > So, I'm afraid this change will make another confusion to users. Could > > > you please apply the new bahavior only for STATX_DIOALIGN? > > > > > > > Well, the issue is that the proposed STATX_DIOALIGN fields cannot represent this > > weird case where DIO reads are allowed but not DIO writes. So the question is > > whether this case actually matters, in which case we should make STATX_DIOALIGN > > distinguish between DIO reads and DIO writes, or whether it's some odd edge case > > that doesn't really matter, in which case we could just fix it or make > > STATX_DIOALIGN report that DIO is unsupported. I was hoping that you had some > > insight here. What sort of applications want DIO reads but not DIO writes? > > Is this common at all? > > I think there's no specific application to use the LFS mode at this > moment, but I'd like to allow DIO read for zoned device which will be > used for Android devices. > So if the zoned device feature becomes widely adopted, then STATX_DIOALIGN will be useless on all Android devices? That sounds undesirable. Are you sure that supporting DIO reads but not DIO writes actually works? Does it not cause problems for existing applications? What we need to do is make a decision about whether this means we should build in a stx_dio_direction field (indicating no support / readonly support / writeonly support / readwrite support) into the API from the beginning. If we don't do that, then I don't think we could simply add such a field later, as the statx_dio_*_align fields will have already been assigned their meaning. I think we'd instead have to "duplicate" the API, with STATX_DIOROALIGN and statx_dio_ro_*_align fields. That seems uglier than building a directional indicator into the API from the beginning. On the other hand, requiring all programs to check stx_dio_direction would add complexity to using the API. Any thoughts on this? - Eric
On Mon, Aug 15, 2022 at 05:55:45PM -0700, Eric Biggers wrote: > On Sat, Jul 30, 2022 at 08:08:26PM -0700, Jaegeuk Kim wrote: > > On 07/25, Eric Biggers wrote: > > > On Sat, Jul 23, 2022 at 07:01:59PM -0700, Jaegeuk Kim wrote: > > > > On 07/22, Eric Biggers wrote: > > > > > From: Eric Biggers <ebiggers@google.com> > > > > > > > > > > Currently, if an f2fs filesystem is mounted with the mode=lfs and > > > > > io_bits mount options, DIO reads are allowed but DIO writes are not. > > > > > Allowing DIO reads but not DIO writes is an unusual restriction, which > > > > > is likely to be surprising to applications, namely any application that > > > > > both reads and writes from a file (using O_DIRECT). This behavior is > > > > > also incompatible with the proposed STATX_DIOALIGN extension to statx. > > > > > Given this, let's drop the support for DIO reads in this configuration. > > > > > > > > IIRC, we allowed DIO reads since applications complained a lower performance. > > > > So, I'm afraid this change will make another confusion to users. Could > > > > you please apply the new bahavior only for STATX_DIOALIGN? > > > > > > > > > > Well, the issue is that the proposed STATX_DIOALIGN fields cannot represent this > > > weird case where DIO reads are allowed but not DIO writes. So the question is > > > whether this case actually matters, in which case we should make STATX_DIOALIGN > > > distinguish between DIO reads and DIO writes, or whether it's some odd edge case > > > that doesn't really matter, in which case we could just fix it or make > > > STATX_DIOALIGN report that DIO is unsupported. I was hoping that you had some > > > insight here. What sort of applications want DIO reads but not DIO writes? > > > Is this common at all? > > > > I think there's no specific application to use the LFS mode at this > > moment, but I'd like to allow DIO read for zoned device which will be > > used for Android devices. > > > > So if the zoned device feature becomes widely adopted, then STATX_DIOALIGN will > be useless on all Android devices? That sounds undesirable. Are you sure that > supporting DIO reads but not DIO writes actually works? Does it not cause > problems for existing applications? What purpose does DIO in only one direction actually serve? All it means is that we're forcibly mixing buffered and direct IO to the same file and that simply never ends well from a data coherency POV. Hence I'd suggest that mixing DIO reads and buffered writes like this ends up exposing uses to the worst of both worlds - all of the problems with none of the benefits... > What we need to do is make a decision about whether this means we should build > in a stx_dio_direction field (indicating no support / readonly support / > writeonly support / readwrite support) into the API from the beginning. If we > don't do that, then I don't think we could simply add such a field later, as the > statx_dio_*_align fields will have already been assigned their meaning. I think > we'd instead have to "duplicate" the API, with STATX_DIOROALIGN and > statx_dio_ro_*_align fields. That seems uglier than building a directional > indicator into the API from the beginning. On the other hand, requiring all > programs to check stx_dio_direction would add complexity to using the API. > > Any thoughts on this? Decide whether partial, single direction DIO serves a useful purpose before trying to work out what is needed in the API to indicate that this sort of crazy will be supported.... Cheers, Dave.
On Aug 16, 2022, at 3:03 AM, Dave Chinner <david@fromorbit.com> wrote: > > On Mon, Aug 15, 2022 at 05:55:45PM -0700, Eric Biggers wrote: >> On Sat, Jul 30, 2022 at 08:08:26PM -0700, Jaegeuk Kim wrote: >>> On 07/25, Eric Biggers wrote: >>>> On Sat, Jul 23, 2022 at 07:01:59PM -0700, Jaegeuk Kim wrote: >>>>> On 07/22, Eric Biggers wrote: >>>>>> From: Eric Biggers <ebiggers@google.com> >>>>>> >>>>>> Currently, if an f2fs filesystem is mounted with the mode=lfs and >>>>>> io_bits mount options, DIO reads are allowed but DIO writes are not. >>>>>> Allowing DIO reads but not DIO writes is an unusual restriction, which >>>>>> is likely to be surprising to applications, namely any application that >>>>>> both reads and writes from a file (using O_DIRECT). This behavior is >>>>>> also incompatible with the proposed STATX_DIOALIGN extension to statx. >>>>>> Given this, let's drop the support for DIO reads in this configuration. >>>>> >>>>> IIRC, we allowed DIO reads since applications complained a lower performance. >>>>> So, I'm afraid this change will make another confusion to users. Could >>>>> you please apply the new bahavior only for STATX_DIOALIGN? >>>>> >>>> >>>> Well, the issue is that the proposed STATX_DIOALIGN fields cannot represent this >>>> weird case where DIO reads are allowed but not DIO writes. So the question is >>>> whether this case actually matters, in which case we should make STATX_DIOALIGN >>>> distinguish between DIO reads and DIO writes, or whether it's some odd edge case >>>> that doesn't really matter, in which case we could just fix it or make >>>> STATX_DIOALIGN report that DIO is unsupported. I was hoping that you had some >>>> insight here. What sort of applications want DIO reads but not DIO writes? >>>> Is this common at all? >>> >>> I think there's no specific application to use the LFS mode at this >>> moment, but I'd like to allow DIO read for zoned device which will be >>> used for Android devices. >>> >> >> So if the zoned device feature becomes widely adopted, then STATX_DIOALIGN will >> be useless on all Android devices? That sounds undesirable. Are you sure that >> supporting DIO reads but not DIO writes actually works? Does it not cause >> problems for existing applications? > > What purpose does DIO in only one direction actually serve? All it > means is that we're forcibly mixing buffered and direct IO to the > same file and that simply never ends well from a data coherency POV. > > Hence I'd suggest that mixing DIO reads and buffered writes like > this ends up exposing uses to the worst of both worlds - all of the > problems with none of the benefits... > >> What we need to do is make a decision about whether this means we should >> build in a stx_dio_direction field (indicating no support / readonly >> support / writeonly support / readwrite support) into the API from the >> beginning. If we don't do that, then I don't think we could simply add >> such a field later, as the statx_dio_*_align fields will have already >> been assigned their meaning. I think we'd instead have to "duplicate" >> the API, with STATX_DIOROALIGN and statx_dio_ro_*_align fields. That >> seems uglier than building a directional indicator into the API from the >> beginning. On the other hand, requiring all programs to check >> stx_dio_direction would add complexity to using the API. >> >> Any thoughts on this? > > Decide whether partial, single direction DIO serves a useful purpose > before trying to work out what is needed in the API to indicate that > this sort of crazy will be supported.... Using read-only O_DIRECT makes sense for backup and other filesystem scanning tools that don't want to pollute the page cache of a system (which may be in use by other programs) while reading many files once. Using interfaces like posix_fadvise(FADV_DONTNEED) to drop file cache afterward is both a hassle and problematic when reading very large files that would push out more important pages from cache before the large file's pages can be dropped. IMHO, this whole discussion is putting the cart before the horse. Changing existing (and useful) IO behavior to accommodate an API that nobody has ever used, and is unlikely to even be widely used, doesn't make sense to me. Most applications won't check or care about the new DIO size fields, since they've lived this long without statx() returning this info, and will just pick a "large enough" size (4KB, 1MB, whatever) that gives them the performance they need. They *WILL* care if the app is suddenly unable to read data from a file in ways that have worked for a long time. Even if apps are modified to check these new DIO size fields, and then try to DIO write to a file in f2fs that doesn't allow it, then f2fs will return an error, which is what it would have done without the statx() changes, so no harm done AFAICS. Even with a more-complex DIO status return that handles a "direction" field (which IMHO is needlessly complex), there is always the potential for a TOCTOU race where a file changes between checking and access, so the userspace code would need to handle this. Cheers, Andreas
On Tue, Aug 16, 2022 at 10:42:29AM -0600, Andreas Dilger wrote: > > IMHO, this whole discussion is putting the cart before the horse. > Changing existing (and useful) IO behavior to accommodate an API that > nobody has ever used, and is unlikely to even be widely used, doesn't > make sense to me. Most applications won't check or care about the new > DIO size fields, since they've lived this long without statx() returning > this info, and will just pick a "large enough" size (4KB, 1MB, whatever) > that gives them the performance they need. They *WILL* care if the app > is suddenly unable to read data from a file in ways that have worked for > a long time. > > Even if apps are modified to check these new DIO size fields, and then > try to DIO write to a file in f2fs that doesn't allow it, then f2fs will > return an error, which is what it would have done without the statx() > changes, so no harm done AFAICS. > > Even with a more-complex DIO status return that handles a "direction" > field (which IMHO is needlessly complex), there is always the potential > for a TOCTOU race where a file changes between checking and access, so > the userspace code would need to handle this. > I'm having trouble making sense of your argument here; you seem to be saying that STATX_DIOALIGN isn't useful, so it doesn't matter if we design it correctly? That line of reasoning is concerning, as it's certainly intended to be useful, and if it's not useful there's no point in adding it. Are there any specific concerns that you have, besides TOCTOU races and the lack of support for read-only DIO? I don't think that TOCTOU races are a real concern here. Generally DIO constraints would only change if the application doing DIO intentionally does something to the file, or if there are changes that involve the filesystem being taken offline, e.g. the filesystem being mounted with significantly different options or being moved to a different block device. And, well, everything else in stat()/statx() is subject to TOCTOU as well, but is still used... - Eric
On 08/15, Eric Biggers wrote: > On Sat, Jul 30, 2022 at 08:08:26PM -0700, Jaegeuk Kim wrote: > > On 07/25, Eric Biggers wrote: > > > On Sat, Jul 23, 2022 at 07:01:59PM -0700, Jaegeuk Kim wrote: > > > > On 07/22, Eric Biggers wrote: > > > > > From: Eric Biggers <ebiggers@google.com> > > > > > > > > > > Currently, if an f2fs filesystem is mounted with the mode=lfs and > > > > > io_bits mount options, DIO reads are allowed but DIO writes are not. > > > > > Allowing DIO reads but not DIO writes is an unusual restriction, which > > > > > is likely to be surprising to applications, namely any application that > > > > > both reads and writes from a file (using O_DIRECT). This behavior is > > > > > also incompatible with the proposed STATX_DIOALIGN extension to statx. > > > > > Given this, let's drop the support for DIO reads in this configuration. > > > > > > > > IIRC, we allowed DIO reads since applications complained a lower performance. > > > > So, I'm afraid this change will make another confusion to users. Could > > > > you please apply the new bahavior only for STATX_DIOALIGN? > > > > > > > > > > Well, the issue is that the proposed STATX_DIOALIGN fields cannot represent this > > > weird case where DIO reads are allowed but not DIO writes. So the question is > > > whether this case actually matters, in which case we should make STATX_DIOALIGN > > > distinguish between DIO reads and DIO writes, or whether it's some odd edge case > > > that doesn't really matter, in which case we could just fix it or make > > > STATX_DIOALIGN report that DIO is unsupported. I was hoping that you had some > > > insight here. What sort of applications want DIO reads but not DIO writes? > > > Is this common at all? > > > > I think there's no specific application to use the LFS mode at this > > moment, but I'd like to allow DIO read for zoned device which will be > > used for Android devices. > > > > So if the zoned device feature becomes widely adopted, then STATX_DIOALIGN will > be useless on all Android devices? That sounds undesirable. Do you have a plan to adopt STATX_DIOALIGN in android? > Are you sure that > supporting DIO reads but not DIO writes actually works? Does it not cause > problems for existing applications? I haven't heard any issue so far. > > What we need to do is make a decision about whether this means we should build > in a stx_dio_direction field (indicating no support / readonly support / > writeonly support / readwrite support) into the API from the beginning. If we > don't do that, then I don't think we could simply add such a field later, as the > statx_dio_*_align fields will have already been assigned their meaning. I think > we'd instead have to "duplicate" the API, with STATX_DIOROALIGN and > statx_dio_ro_*_align fields. That seems uglier than building a directional > indicator into the API from the beginning. On the other hand, requiring all > programs to check stx_dio_direction would add complexity to using the API. > > Any thoughts on this? I haven't seen the details of the implementation tho, why not supporting it only if filesystem has the same DIO RW policy? > > - Eric
On Fri, Aug 19, 2022 at 05:06:06PM -0700, Jaegeuk Kim wrote: > On 08/15, Eric Biggers wrote: > > On Sat, Jul 30, 2022 at 08:08:26PM -0700, Jaegeuk Kim wrote: > > > On 07/25, Eric Biggers wrote: > > > > On Sat, Jul 23, 2022 at 07:01:59PM -0700, Jaegeuk Kim wrote: > > > > > On 07/22, Eric Biggers wrote: > > > > > > From: Eric Biggers <ebiggers@google.com> > > > > > > > > > > > > Currently, if an f2fs filesystem is mounted with the mode=lfs and > > > > > > io_bits mount options, DIO reads are allowed but DIO writes are not. > > > > > > Allowing DIO reads but not DIO writes is an unusual restriction, which > > > > > > is likely to be surprising to applications, namely any application that > > > > > > both reads and writes from a file (using O_DIRECT). This behavior is > > > > > > also incompatible with the proposed STATX_DIOALIGN extension to statx. > > > > > > Given this, let's drop the support for DIO reads in this configuration. > > > > > > > > > > IIRC, we allowed DIO reads since applications complained a lower performance. > > > > > So, I'm afraid this change will make another confusion to users. Could > > > > > you please apply the new bahavior only for STATX_DIOALIGN? > > > > > > > > > > > > > Well, the issue is that the proposed STATX_DIOALIGN fields cannot represent this > > > > weird case where DIO reads are allowed but not DIO writes. So the question is > > > > whether this case actually matters, in which case we should make STATX_DIOALIGN > > > > distinguish between DIO reads and DIO writes, or whether it's some odd edge case > > > > that doesn't really matter, in which case we could just fix it or make > > > > STATX_DIOALIGN report that DIO is unsupported. I was hoping that you had some > > > > insight here. What sort of applications want DIO reads but not DIO writes? > > > > Is this common at all? > > > > > > I think there's no specific application to use the LFS mode at this > > > moment, but I'd like to allow DIO read for zoned device which will be > > > used for Android devices. > > > > > > > So if the zoned device feature becomes widely adopted, then STATX_DIOALIGN will > > be useless on all Android devices? That sounds undesirable. > > Do you have a plan to adopt STATX_DIOALIGN in android? Nothing specific, but statx() is among the system calls that are supported by Android's libc and that apps are allowed to use. So STATX_DIOALIGN would become available as well. I'd prefer if it actually worked properly if apps, or Android system components, do actually try to use it (or need to use it)... > > What we need to do is make a decision about whether this means we should build > > in a stx_dio_direction field (indicating no support / readonly support / > > writeonly support / readwrite support) into the API from the beginning. If we > > don't do that, then I don't think we could simply add such a field later, as the > > statx_dio_*_align fields will have already been assigned their meaning. I think > > we'd instead have to "duplicate" the API, with STATX_DIOROALIGN and > > statx_dio_ro_*_align fields. That seems uglier than building a directional > > indicator into the API from the beginning. On the other hand, requiring all > > programs to check stx_dio_direction would add complexity to using the API. > > > > Any thoughts on this? > > I haven't seen the details of the implementation tho, why not supporting it > only if filesystem has the same DIO RW policy? As I've mentioned, we could of course make STATX_DIOALIGN report that DIO is unsupported when the DIO support is read-only. The thing that confuses me based on the responses so far is that there seem to be two camps of people: (1) people who really want STATX_DIOALIGN, and who don't think that read-only DIO support should exist so they don't want STATX_DIOALIGN to support it; and (2) people who feel that read-only DIO support is perfectly reasonable and useful, and who don't care whether STATX_DIOALIGN supports it because they don't care about STATX_DIOALIGN in the first place. While both camps seem to agree that STATX_DIOALIGN shouldn't support read-only DIO, it is for totally contradictory reasons, so it's not very convincing. We should ensure that we have rock-solid reasoning before committing to a new UAPI that will have to be permanently supported... - Eric
On Mon, Aug 15, 2022 at 05:55:45PM -0700, Eric Biggers wrote: > So if the zoned device feature becomes widely adopted, then STATX_DIOALIGN will > be useless on all Android devices? That sounds undesirable. Are you sure that We just need to fix f2fs to support direct I/O on zone devices. There is not good reason not to support it, in fact the way how zoned devices requires appends with the Zone Append semantics makes direct I/O way safer than how f2fs does direct I/O currently on non-zoned devices. Until then just supporting direct I/O reads on zoned devices for f2fs seems like a really bad choice given that it will lead to nasty cache incoherency.
On Aug 19, 2022, at 5:09 PM, Eric Biggers <ebiggers@kernel.org> wrote: > > On Tue, Aug 16, 2022 at 10:42:29AM -0600, Andreas Dilger wrote: >> >> IMHO, this whole discussion is putting the cart before the horse. >> Changing existing (and useful) IO behavior to accommodate an API that >> nobody has ever used, and is unlikely to even be widely used, doesn't >> make sense to me. Most applications won't check or care about the new >> DIO size fields, since they've lived this long without statx() returning >> this info, and will just pick a "large enough" size (4KB, 1MB, whatever) >> that gives them the performance they need. They *WILL* care if the app >> is suddenly unable to read data from a file in ways that have worked for >> a long time. >> >> Even if apps are modified to check these new DIO size fields, and then >> try to DIO write to a file in f2fs that doesn't allow it, then f2fs will >> return an error, which is what it would have done without the statx() >> changes, so no harm done AFAICS. >> >> Even with a more-complex DIO status return that handles a "direction" >> field (which IMHO is needlessly complex), there is always the potential >> for a TOCTOU race where a file changes between checking and access, so >> the userspace code would need to handle this. > > I'm having trouble making sense of your argument here; you seem to be saying > that STATX_DIOALIGN isn't useful, so it doesn't matter if we design it > correctly? That line of reasoning is concerning, as it's certainly intended > to be useful, and if it's not useful there's no point in adding it. > > Are there any specific concerns that you have, besides TOCTOU races and the > lack of support for read-only DIO? My main concern is disabling useful functionality that exists today to appease the new DIO size API. Whether STATX_DIOALIGN will become widely used by applications or not is hard to say at this point. If there were separate STATX_DIOREAD and STATX_DIOWRITE flags in the returned data, and the alignment is provided as it is today, that would be enough IMHO to address the original use case without significant complexity. > I don't think that TOCTOU races are a real concern here. Generally DIO > constraints would only change if the application doing DIO intentionally does > something to the file, or if there are changes that involve the filesystem > being taken offline, e.g. the filesystem being mounted with significantly > different options or being moved to a different block device. And, well, > everything else in stat()/statx() is subject to TOCTOU as well, but is still > used... I was thinking of background filesystem operations like compression, LVM migration to new storage with a different sector size, etc. that may change the DIO characteristics of the file even while it is open. Not that I think this will happen frequently, but it is possible, and applications shouldn't explode if the DIO parameters change and they get an error. Cheers, Andreas
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c index 5e5c97fccfb4ee..ad0212848a1ab9 100644 --- a/fs/f2fs/file.c +++ b/fs/f2fs/file.c @@ -823,7 +823,6 @@ static inline bool f2fs_force_buffered_io(struct inode *inode, struct kiocb *iocb, struct iov_iter *iter) { struct f2fs_sb_info *sbi = F2FS_I_SB(inode); - int rw = iov_iter_rw(iter); if (!fscrypt_dio_supported(inode)) return true; @@ -841,7 +840,7 @@ static inline bool f2fs_force_buffered_io(struct inode *inode, */ if (f2fs_sb_has_blkzoned(sbi)) return true; - if (f2fs_lfs_mode(sbi) && (rw == WRITE)) { + if (f2fs_lfs_mode(sbi)) { if (block_unaligned_IO(inode, iocb, iter)) return true; if (F2FS_IO_ALIGNED(sbi))