| Message ID | 20210909032007.18353-1-liuyuntao10@huawei.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] fix judgment error in shmem_is_huge() | expand |
On Thu, Sep 09, 2021 at 11:20:07AM +0800, Liu Yuntao wrote: > In the case of SHMEM_HUGE_WITHIN_SIZE, the page index is not rounded > up correctly. When the page index points to the first page in a huge > page, round_up() cannot bring it to the end of the huge page, but > to the end of the previous one. > > an example: > HPAGE_PMD_NR on my machine is 512(2 MB huge page size). > After allcoating a 3000 KB buffer, I access it at location 2050 KB. > In shmem_is_huge(), the corresponding index happens to be 512. > After rounded up by HPAGE_PMD_NR, it will still be 512 which is > smaller than i_size, and shmem_is_huge() will return true. > As a result, my buffer takes an additional huge page, and that > shouldn't happen when shmem_enabled is set to within_size. > > Fixes: f3f0e1d2150b2b ("khugepaged: add support of collapse for tmpfs/shmem pages") > Signed-off-by: Liu Yuntao <liuyuntao10@huawei.com> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
On Thu, 9 Sep 2021, Liu Yuntao wrote: > In the case of SHMEM_HUGE_WITHIN_SIZE, the page index is not rounded > up correctly. When the page index points to the first page in a huge > page, round_up() cannot bring it to the end of the huge page, but > to the end of the previous one. > > an example: > HPAGE_PMD_NR on my machine is 512(2 MB huge page size). > After allcoating a 3000 KB buffer, I access it at location 2050 KB. Your example is certainly helpful, but weird! It's not impossible, but wouldn't it be easier to understand if you said "2048 KB" there? > In shmem_is_huge(), the corresponding index happens to be 512. > After rounded up by HPAGE_PMD_NR, it will still be 512 which is > smaller than i_size, and shmem_is_huge() will return true. > As a result, my buffer takes an additional huge page, and that > shouldn't happen when shmem_enabled is set to within_size. A colleague very recently opened my eyes to within_size on shmem_enabled: I've always been dubious of both, but they can work quite well together. > > Fixes: f3f0e1d2150b2b ("khugepaged: add support of collapse for tmpfs/shmem pages") > Signed-off-by: Liu Yuntao <liuyuntao10@huawei.com> Thanks, with a nice simplification from Kirill. Acked-by: Hugh Dickins <hughd@google.com> Ignore the comment I've added below - it's not worth worrying about. > --- > V1->V2: > add simplification of the condition after round_up() > --- > mm/shmem.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/mm/shmem.c b/mm/shmem.c > index 88742953532c..b5860f4a2738 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c > @@ -490,9 +490,9 @@ bool shmem_is_huge(struct vm_area_struct *vma, > case SHMEM_HUGE_ALWAYS: > return true; > case SHMEM_HUGE_WITHIN_SIZE: > - index = round_up(index, HPAGE_PMD_NR); > + index = round_up(index + 1, HPAGE_PMD_NR); Even without your change, I notice now that there's a possibility of index wrapping to 0 on 32-bit architecture here. But nothing goes terribly wrong in that case: it is not worth worrying about here. > i_size = round_up(i_size_read(inode), PAGE_SIZE); > - if (i_size >= HPAGE_PMD_SIZE && (i_size >> PAGE_SHIFT) >= index) > + if (i_size >> PAGE_SHIFT >= index) > return true; > fallthrough; > case SHMEM_HUGE_ADVISE: > -- > 2.23.0
On Fri, 24 Sep 2021, Hugh Dickins wrote: > On Thu, 9 Sep 2021, Liu Yuntao wrote: > > > In the case of SHMEM_HUGE_WITHIN_SIZE, the page index is not rounded > > up correctly. When the page index points to the first page in a huge > > page, round_up() cannot bring it to the end of the huge page, but > > to the end of the previous one. > > > > an example: > > HPAGE_PMD_NR on my machine is 512(2 MB huge page size). > > After allcoating a 3000 KB buffer, I access it at location 2050 KB. > > Your example is certainly helpful, but weird! It's not impossible, > but wouldn't it be easier to understand if you said "2048 KB" there? > > > In shmem_is_huge(), the corresponding index happens to be 512. > > After rounded up by HPAGE_PMD_NR, it will still be 512 which is > > smaller than i_size, and shmem_is_huge() will return true. > > As a result, my buffer takes an additional huge page, and that > > shouldn't happen when shmem_enabled is set to within_size. > > A colleague very recently opened my eyes to within_size on shmem_enabled: > I've always been dubious of both, but they can work quite well together. > > > > > Fixes: f3f0e1d2150b2b ("khugepaged: add support of collapse for tmpfs/shmem pages") > > Signed-off-by: Liu Yuntao <liuyuntao10@huawei.com> > > Thanks, with a nice simplification from Kirill. > > Acked-by: Hugh Dickins <hughd@google.com> Andrew has just sent this on to Linus - thanks - and that's fine: no need to get in the way of that. But since replying, I have remembered more history, and there is something that we need to be aware of. Whereas to you this is a straightforward off-by-one (or off-by-page) fix, it also results in a significant change in behaviour - I'd say usually for the better, but some might be surprised. This patch has Kirill's Ack and my Ack, and I hope and believe that we can get away with the change in behaviour: but let's be aware of it. The change that concerns me is when, for example, copying a large file into a huge=within_size tmpfs (or more generally, just writing to the file by appending at EOF in the usual way). With the old WITHIN_SIZE code, the first 2MB was allocated in small pages, then subsequent 2MB extents were allocated with huge pages; including the final extent, even if it only needed a single byte. I always thought that was very clunky behaviour, the small pages coming at the wrong end of the file; and that's why I was dubious about it as a sensible filesystem mount option. But I was under the impression that it was the intended behaviour. With your new WITHIN_SIZE code, all those appending allocations are outside i_size, and the whole file is allocated in small pages. (Then maybe later on khugepaged can assemble huge pages for it.) Your patch makes within_size more sensible than it was for pre-sized files (and I think it's fair to say that the majority of files in shmem's internal mount, subject to thp/shmem_enabled, are likely to be fixed-size files); and better-defined than it used to be on growing files, but they won't get the huge pages they used to. Hugh
On Sat, 25 Sep 2021, Hugh Dickins wrote: > On Fri, 24 Sep 2021, Hugh Dickins wrote: > > On Thu, 9 Sep 2021, Liu Yuntao wrote: > > > > > In the case of SHMEM_HUGE_WITHIN_SIZE, the page index is not rounded > > > up correctly. When the page index points to the first page in a huge > > > page, round_up() cannot bring it to the end of the huge page, but > > > to the end of the previous one. > > > > > > an example: > > > HPAGE_PMD_NR on my machine is 512(2 MB huge page size). > > > After allcoating a 3000 KB buffer, I access it at location 2050 KB. > > > > Your example is certainly helpful, but weird! It's not impossible, > > but wouldn't it be easier to understand if you said "2048 KB" there? I wanted to emphasize that access to any bit in the first page will trigger this problem, so I didn't use "2048 KB". > > > > > In shmem_is_huge(), the corresponding index happens to be 512. > > > After rounded up by HPAGE_PMD_NR, it will still be 512 which is > > > smaller than i_size, and shmem_is_huge() will return true. > > > As a result, my buffer takes an additional huge page, and that > > > shouldn't happen when shmem_enabled is set to within_size. > > > > A colleague very recently opened my eyes to within_size on shmem_enabled: > > I've always been dubious of both, but they can work quite well together. > > > > > > > > Fixes: f3f0e1d2150b2b ("khugepaged: add support of collapse for tmpfs/shmem pages") > > > Signed-off-by: Liu Yuntao <liuyuntao10@huawei.com> > > > > Thanks, with a nice simplification from Kirill. > > > > Acked-by: Hugh Dickins <hughd@google.com> > > Andrew has just sent this on to Linus - thanks - and that's fine: > no need to get in the way of that. > > But since replying, I have remembered more history, and there is > something that we need to be aware of. > > Whereas to you this is a straightforward off-by-one (or off-by-page) > fix, it also results in a significant change in behaviour - I'd say > usually for the better, but some might be surprised. This patch has > Kirill's Ack and my Ack, and I hope and believe that we can get away > with the change in behaviour: but let's be aware of it. > > The change that concerns me is when, for example, copying a large > file into a huge=within_size tmpfs (or more generally, just writing > to the file by appending at EOF in the usual way). > > With the old WITHIN_SIZE code, the first 2MB was allocated in small > pages, then subsequent 2MB extents were allocated with huge pages; > including the final extent, even if it only needed a single byte. > > I always thought that was very clunky behaviour, the small pages > coming at the wrong end of the file; and that's why I was dubious > about it as a sensible filesystem mount option. But I was under > the impression that it was the intended behaviour. > > With your new WITHIN_SIZE code, all those appending allocations > are outside i_size, and the whole file is allocated in small pages. > (Then maybe later on khugepaged can assemble huge pages for it.) > > Your patch makes within_size more sensible than it was for pre-sized > files (and I think it's fair to say that the majority of files in > shmem's internal mount, subject to thp/shmem_enabled, are likely to > be fixed-size files); and better-defined than it used to be on > growing files, but they won't get the huge pages they used to. Although my patch changes shmem's behaviour, it makes shmem consistent with the documentation. I think with the new code, it will be easier for our users to understand. > > Hugh
On Sun, 26 Sep 2021, liuyuntao wrote: > On Sat, 25 Sep 2021, Hugh Dickins wrote: > > On Fri, 24 Sep 2021, Hugh Dickins wrote: > > > On Thu, 9 Sep 2021, Liu Yuntao wrote: > > > > > > > In the case of SHMEM_HUGE_WITHIN_SIZE, the page index is not rounded > > > > up correctly. When the page index points to the first page in a huge > > > > page, round_up() cannot bring it to the end of the huge page, but > > > > to the end of the previous one. > > > > > > > > an example: > > > > HPAGE_PMD_NR on my machine is 512(2 MB huge page size). > > > > After allcoating a 3000 KB buffer, I access it at location 2050 KB. > > > > > > Your example is certainly helpful, but weird! It's not impossible, > > > but wouldn't it be easier to understand if you said "2048 KB" there? > > I wanted to emphasize that access to any bit in the first page will > trigger this problem, so I didn't use "2048 KB". Okay, thanks, I see your point now. (And I have to admit that, in my confusion, I had thought 2050 KB would be index 514 - of course not!) > > > > In shmem_is_huge(), the corresponding index happens to be 512. ... > > Your patch makes within_size more sensible than it was for pre-sized > > files (and I think it's fair to say that the majority of files in > > shmem's internal mount, subject to thp/shmem_enabled, are likely to > > be fixed-size files); and better-defined than it used to be on > > growing files, but they won't get the huge pages they used to. > > Although my patch changes shmem's behaviour, it makes shmem consistent > with the documentation. I think with the new code, it will be easier > for our users to understand. Yes, I do agree with you. But the change in behaviour when appending at EOF is significant, and needed to be called out - I think none of quite realized that effect at first. Hugh
diff --git a/mm/shmem.c b/mm/shmem.c index 88742953532c..b5860f4a2738 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -490,9 +490,9 @@ bool shmem_is_huge(struct vm_area_struct *vma, case SHMEM_HUGE_ALWAYS: return true; case SHMEM_HUGE_WITHIN_SIZE: - index = round_up(index, HPAGE_PMD_NR); + index = round_up(index + 1, HPAGE_PMD_NR); i_size = round_up(i_size_read(inode), PAGE_SIZE); - if (i_size >= HPAGE_PMD_SIZE && (i_size >> PAGE_SHIFT) >= index) + if (i_size >> PAGE_SHIFT >= index) return true; fallthrough; case SHMEM_HUGE_ADVISE:
In the case of SHMEM_HUGE_WITHIN_SIZE, the page index is not rounded up correctly. When the page index points to the first page in a huge page, round_up() cannot bring it to the end of the huge page, but to the end of the previous one. an example: HPAGE_PMD_NR on my machine is 512(2 MB huge page size). After allcoating a 3000 KB buffer, I access it at location 2050 KB. In shmem_is_huge(), the corresponding index happens to be 512. After rounded up by HPAGE_PMD_NR, it will still be 512 which is smaller than i_size, and shmem_is_huge() will return true. As a result, my buffer takes an additional huge page, and that shouldn't happen when shmem_enabled is set to within_size. Fixes: f3f0e1d2150b2b ("khugepaged: add support of collapse for tmpfs/shmem pages") Signed-off-by: Liu Yuntao <liuyuntao10@huawei.com> --- V1->V2: add simplification of the condition after round_up() --- mm/shmem.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)