| Message ID | 163456863216.2614702.6384850026368833133.stgit@warthog.procyon.org.uk (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | fscache: Rewrite index API and management system | expand |
On Mon, 2021-10-18 at 15:50 +0100, David Howells wrote: > Under some circumstances, filemap_read() will allocate sufficient pages to > read to the end of the file, call readahead/readpages on them and copy the > data over - and then it will allocate another page at the EOF and call > readpage on that and then ignore it. This is unnecessary and a waste of > time and resources. > > filemap_read() *does* check for this, but only after it has already done > the allocation and I/O. Fix this by checking before calling > filemap_get_pages() also. > > Signed-off-by: David Howells <dhowells@redhat.com> > Acked-by: Kent Overstreet <kent.overstreet@gmail.com> > cc: Matthew Wilcox (Oracle) <willy@infradead.org> > cc: linux-mm@kvack.org > cc: linux-fsdevel@vger.kernel.org > Link: https://lore.kernel.org/r/160588481358.3465195.16552616179674485179.stgit@warthog.procyon.org.uk/ > --- > > mm/filemap.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/mm/filemap.c b/mm/filemap.c > index dae481293b5d..c0cdc44c844e 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -2625,6 +2625,10 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter, > if ((iocb->ki_flags & IOCB_WAITQ) && already_read) > iocb->ki_flags |= IOCB_NOWAIT; > > + isize = i_size_read(inode); > + if (unlikely(iocb->ki_pos >= isize)) > + goto put_pages; > + > error = filemap_get_pages(iocb, iter, &pvec); > if (error < 0) > break; > > I would wager that it's worth checking for this. I imagine read calls beyond EOF are common enough that it's probably helpful to optimize that case: Acked-by: Jeff Layton <jlayton@redhat.com>
On Mon, Oct 18, 2021 at 03:50:32PM +0100, David Howells wrote: > @@ -2625,6 +2625,10 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter, > if ((iocb->ki_flags & IOCB_WAITQ) && already_read) > iocb->ki_flags |= IOCB_NOWAIT; > > + isize = i_size_read(inode); > + if (unlikely(iocb->ki_pos >= isize)) > + goto put_pages; > + Is there a good reason to assign to isize here? I'd rather not, because it complicates analysis, and a later change might look at the isize read here, not realising it was a racy use. So I'd rather see: if (unlikely(iocb->ki_pos >= i_size_read(inode))) goto put_pages;
Matthew Wilcox <willy@infradead.org> wrote: > > + isize = i_size_read(inode); > > + if (unlikely(iocb->ki_pos >= isize)) > > + goto put_pages; > > + > > Is there a good reason to assign to isize here? I'd rather not, > because it complicates analysis, and a later change might look at > the isize read here, not realising it was a racy use. So I'd > rather see: If we don't set isize, the loop will never end. Actually, maybe we can just break out at that point rather than going to put_pages. David
On Tue, Oct 19, 2021 at 07:48:15PM +0100, David Howells wrote: > Matthew Wilcox <willy@infradead.org> wrote: > > > > + isize = i_size_read(inode); > > > + if (unlikely(iocb->ki_pos >= isize)) > > > + goto put_pages; > > > + > > > > Is there a good reason to assign to isize here? I'd rather not, > > because it complicates analysis, and a later change might look at > > the isize read here, not realising it was a racy use. So I'd > > rather see: > > If we don't set isize, the loop will never end. Actually, maybe we can just > break out at that point rather than going to put_pages. Umm, yes, of course. Sorry. It makes more sense to just break because we haven't got any pages, so putting pages that we haven't got seems unnecessary. >
diff --git a/mm/filemap.c b/mm/filemap.c index dae481293b5d..c0cdc44c844e 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2625,6 +2625,10 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter, if ((iocb->ki_flags & IOCB_WAITQ) && already_read) iocb->ki_flags |= IOCB_NOWAIT; + isize = i_size_read(inode); + if (unlikely(iocb->ki_pos >= isize)) + goto put_pages; + error = filemap_get_pages(iocb, iter, &pvec); if (error < 0) break;