| Message ID | 163472463105.3126792.7056099385135786492.stgit@warthog.procyon.org.uk (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] mm: Stop filemap_read() from grabbing a superfluous page | expand |
On Wed, 2021-10-20 at 11:10 +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. > > Changes: > v2) Break out of the loop immediately rather than going to put_pages (the > pvec is unoccupied). Setting isize is then unnecessary. > > Signed-off-by: David Howells <dhowells@redhat.com> > cc: Kent Overstreet <kent.overstreet@gmail.com> > cc: Matthew Wilcox (Oracle) <willy@infradead.org> > cc: Andrew Morton <akpm@linux-foundation.org> > cc: Jeff Layton <jlayton@redhat.com> > 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/ > Link: https://lore.kernel.org/r/163456863216.2614702.6384850026368833133.stgit@warthog.procyon.org.uk/ > --- > > mm/filemap.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/mm/filemap.c b/mm/filemap.c > index dae481293b5d..e50be519f6a4 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -2625,6 +2625,9 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter, > if ((iocb->ki_flags & IOCB_WAITQ) && already_read) > iocb->ki_flags |= IOCB_NOWAIT; > > + if (unlikely(iocb->ki_pos >= i_size_read(inode))) > + break; > + > error = filemap_get_pages(iocb, iter, &pvec); > if (error < 0) > break; > > Even better. Acked-by: Jeff Layton <jlayton@kernel.org>
On Wed, Oct 20, 2021 at 11:10:31AM +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. > > Changes: > v2) Break out of the loop immediately rather than going to put_pages (the > pvec is unoccupied). Setting isize is then unnecessary. > > Signed-off-by: David Howells <dhowells@redhat.com> > cc: Kent Overstreet <kent.overstreet@gmail.com> > cc: Matthew Wilcox (Oracle) <willy@infradead.org> > cc: Andrew Morton <akpm@linux-foundation.org> > cc: Jeff Layton <jlayton@redhat.com> > 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/ > Link: https://lore.kernel.org/r/163456863216.2614702.6384850026368833133.stgit@warthog.procyon.org.uk/ Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
diff --git a/mm/filemap.c b/mm/filemap.c index dae481293b5d..e50be519f6a4 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2625,6 +2625,9 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter, if ((iocb->ki_flags & IOCB_WAITQ) && already_read) iocb->ki_flags |= IOCB_NOWAIT; + if (unlikely(iocb->ki_pos >= i_size_read(inode))) + break; + error = filemap_get_pages(iocb, iter, &pvec); if (error < 0) break;
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. Changes: v2) Break out of the loop immediately rather than going to put_pages (the pvec is unoccupied). Setting isize is then unnecessary. Signed-off-by: David Howells <dhowells@redhat.com> cc: Kent Overstreet <kent.overstreet@gmail.com> cc: Matthew Wilcox (Oracle) <willy@infradead.org> cc: Andrew Morton <akpm@linux-foundation.org> cc: Jeff Layton <jlayton@redhat.com> 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/ Link: https://lore.kernel.org/r/163456863216.2614702.6384850026368833133.stgit@warthog.procyon.org.uk/ --- mm/filemap.c | 3 +++ 1 file changed, 3 insertions(+)