[PATCH] [PATCH V2] mm/filemap: fix page end in filemap_get_read_batch

[PATCH] [PATCH V2] mm/filemap: fix page end in filemap_get_read_batch

[coolqyj]

From: Qian Yingjin <qian@ddn.com>

I was running traces of the read code against an RAID storage
system to understand why read requests were being misaligned
against the underlying RAID strips. I found that the page end
offset calculation in filemap_get_read_batch() was off by one.

When a read is submitted with end offset 1048575, then it
calculates the end page for read of 256 when it should be 255.
"last_index" is the index of the page beyond the end of the read
and it should be skipped when get a batch of pages for read in
@filemap_get_read_batch().

The below simple patch fixes the problem. This code was introduced
in kernel 5.12.

Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read")

Signed-off-by: Qian Yingjin <qian@ddn.com>
---
 mm/filemap.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index c4d4ace9cc70..0e20a8d6dd93 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2588,18 +2588,19 @@ static int filemap_get_pages(struct kiocb *iocb, struct iov_iter *iter,
 	struct folio *folio;
 	int err = 0;
 
+	/* "last_index" is the index of the page beyond the end of the read */
 	last_index = DIV_ROUND_UP(iocb->ki_pos + iter->count, PAGE_SIZE);
 retry:
 	if (fatal_signal_pending(current))
 		return -EINTR;
 
-	filemap_get_read_batch(mapping, index, last_index, fbatch);
+	filemap_get_read_batch(mapping, index, last_index - 1, fbatch);
 	if (!folio_batch_count(fbatch)) {
 		if (iocb->ki_flags & IOCB_NOIO)
 			return -EAGAIN;
 		page_cache_sync_readahead(mapping, ra, filp, index,
 				last_index - index);
-		filemap_get_read_batch(mapping, index, last_index, fbatch);
+		filemap_get_read_batch(mapping, index, last_index - 1, fbatch);
 	}
 	if (!folio_batch_count(fbatch)) {
 		if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_WAITQ))
-- 
2.34.1

Re: [PATCH] [PATCH V2] mm/filemap: fix page end in filemap_get_read_batch

[Matthew Wilcox]

On Wed, Feb 08, 2023 at 10:24:00AM +0800, coolqyj@163.com wrote:
> From: Qian Yingjin <qian@ddn.com>
> 
> I was running traces of the read code against an RAID storage
> system to understand why read requests were being misaligned
> against the underlying RAID strips. I found that the page end
> offset calculation in filemap_get_read_batch() was off by one.
> 
> When a read is submitted with end offset 1048575, then it
> calculates the end page for read of 256 when it should be 255.
> "last_index" is the index of the page beyond the end of the read
> and it should be skipped when get a batch of pages for read in
> @filemap_get_read_batch().
> 
> The below simple patch fixes the problem. This code was introduced
> in kernel 5.12.
> 
> Fixes: cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read")
> Signed-off-by: Qian Yingjin <qian@ddn.com>

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: stable@vger.kernel.org

> ---
>  mm/filemap.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index c4d4ace9cc70..0e20a8d6dd93 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2588,18 +2588,19 @@ static int filemap_get_pages(struct kiocb *iocb, struct iov_iter *iter,
>  	struct folio *folio;
>  	int err = 0;
>  
> +	/* "last_index" is the index of the page beyond the end of the read */
>  	last_index = DIV_ROUND_UP(iocb->ki_pos + iter->count, PAGE_SIZE);
>  retry:
>  	if (fatal_signal_pending(current))
>  		return -EINTR;
>  
> -	filemap_get_read_batch(mapping, index, last_index, fbatch);
> +	filemap_get_read_batch(mapping, index, last_index - 1, fbatch);
>  	if (!folio_batch_count(fbatch)) {
>  		if (iocb->ki_flags & IOCB_NOIO)
>  			return -EAGAIN;
>  		page_cache_sync_readahead(mapping, ra, filp, index,
>  				last_index - index);
> -		filemap_get_read_batch(mapping, index, last_index, fbatch);
> +		filemap_get_read_batch(mapping, index, last_index - 1, fbatch);
>  	}
>  	if (!folio_batch_count(fbatch)) {
>  		if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_WAITQ))
> -- 
> 2.34.1
>