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@xxxxxxxxxx> > Acked-by: Kent Overstreet <kent.overstreet@xxxxxxxxx> > cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> > cc: linux-mm@xxxxxxxxx > cc: linux-fsdevel@xxxxxxxxxxxxxxx > Link: https://lore.kernel.org/r/160588481358.3465195.16552616179674485179.stgit@xxxxxxxxxxxxxxxxxxxxxx/ > --- > > 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@xxxxxxxxxx>
