linux-kernel - Re: fs/direct-io.c: don't try to allocate more than BIO_MAX

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Date:	Fri, 14 Jan 2011 09:44:33 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	David Dillow <dillowda@...l.gov>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: fs/direct-io.c: don't try to allocate more than BIO_MAX_PAGES in a bio

David Dillow <dillowda@...l.gov> writes:

> When using devices that support max_segments > BIO_MAX_PAGES (256),
> direct IO tries to allocate a bio with more pages than allowed, which
> leads to an oops in dio_bio_alloc(). Clamp that request to the supported
> maximum.
>
> Signed-off-by: David Dillow <dillowda@...l.gov>
> --
> dio_bio_alloc() doesn't check the result of bio_alloc(), so it
> dereferences a NULL pointer. bio_alloc(GFP_KERNEL, ...) doesn't fail
> unless it gets called for an invalid number of pages, so it seems a bit
> like overkill to check for failure in dio_bio_alloc(), though it would
> have saved me some time tracking this down.

We should always be checking return values.  Mind respinning the patch
to include that?

> diff --git a/fs/direct-io.c b/fs/direct-io.c
> index 85882f6..9eb0553 100644
> --- a/fs/direct-io.c
> +++ b/fs/direct-io.c
> @@ -583,6 +583,7 @@ static int dio_new_bio(struct dio *dio, sector_t start_sector)
>  		goto out;
>  	sector = start_sector << (dio->blkbits - 9);
>  	nr_pages = min(dio->pages_in_io, bio_get_nr_vecs(dio->map_bh.b_bdev));
> +	nr_pages = min(nr_pages, BIO_MAX_PAGES);

This looks fine.  Thanks for tracking it down!

Cheers,
Jeff
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