[PATCH v2 alt 1] fs/direct-io.c: don't try to allocate more thanBIO_MAX_PAGES in a bio

[David Dillow]

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 the request to the supported
maximum, and return an error to catch future cases.
    
Signed-off-by: David Dillow <dillowda@xxxxxxxx>
--
On Fri, 2011-01-14 at 09:44 -0500, Jeff Moyer wrote:
> David Dillow <dillowda@xxxxxxxx> writes:
> > 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?

Here's the first alternate, that just checks the return value. I think I
prefer the other one, though.


diff --git a/fs/direct-io.c b/fs/direct-io.c
index 85882f6..9fdbf99 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -332,6 +332,8 @@ dio_bio_alloc(struct dio *dio, struct block_device *bdev,
 	struct bio *bio;
 
 	bio = bio_alloc(GFP_KERNEL, nr_vecs);
+	if (!bio)
+		return -ENOMEM;
 
 	bio->bi_bdev = bdev;
 	bio->bi_sector = first_sector;
@@ -583,6 +585,7 @@ 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);
 	BUG_ON(nr_pages <= 0);
 	ret = dio_bio_alloc(dio, dio->map_bh.b_bdev, sector, nr_pages);
 	dio->boundary = 0;




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