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* [RFC PATCH] mm: Stop filemap_read() from grabbing a superfluous page
@ 2021-10-07 12:01 David Howells
  2021-10-13 15:58 ` Kent Overstreet
  0 siblings, 1 reply; 2+ messages in thread
From: David Howells @ 2021-10-07 12:01 UTC (permalink / raw)
  To: kent.overstreet, willy; +Cc: dhowells, linux-mm, linux-fsdevel, linux-kernel

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(+)

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;



^ permalink raw reply	[flat|nested] 2+ messages in thread

* Re: [RFC PATCH] mm: Stop filemap_read() from grabbing a superfluous page
  2021-10-07 12:01 [RFC PATCH] mm: Stop filemap_read() from grabbing a superfluous page David Howells
@ 2021-10-13 15:58 ` Kent Overstreet
  0 siblings, 0 replies; 2+ messages in thread
From: Kent Overstreet @ 2021-10-13 15:58 UTC (permalink / raw)
  To: David Howells; +Cc: willy, linux-mm, linux-fsdevel, linux-kernel

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>

^ permalink raw reply	[flat|nested] 2+ messages in thread

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