| Message ID | 163360810881.1636291.17477809397516812670.stgit@warthog.procyon.org.uk (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [RFC] mm: Stop filemap_read() from grabbing a superfluous page | expand |
On Thu, Oct 07, 2021 at 01:01:48PM +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> > cc: 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/ Acked-by: Kent Overstreet <kent.overstreet@gmail.com>
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;
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> cc: 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(+)