Re: [dm-devel] regression: data corruption with ext4 on LUKS on nvme with torvalds master

[Mikulas Patocka <mpatocka@redhat.com>]

> On 5/13/21 7:15 AM, Theodore Ts'o wrote:
> > On Thu, May 13, 2021 at 06:42:22PM +0900, Changheun Lee wrote:
> >>
> >> Problem might be casued by exhausting of memory. And memory exhausting
> >> would be caused by setting of small bio_max_size. Actually it was not
> >> reproduced in my VM environment at first. But, I reproduced same problem
> >> when bio_max_size is set with 8KB forced. Too many bio allocation would
> >> be occurred by setting of 8KB bio_max_size.
> > 
> > Hmm... I'm not sure how to align your diagnosis with the symptoms in
> > the bug report.  If we were limited by memory, that should slow down
> > the I/O, but we should still be making forward progress, no?  And a
> > forced reboot should not result in data corruption, unless maybe there
> > was a missing check for a failed memory allocation, causing data to be
> > written to the wrong location, a missing error check leading to the
> > block or file system layer not noticing that a write had failed
> > (although again, memory exhaustion should not lead to failed writes;
> > it might slow us down, sure, but if writes are being failed, something
> > is Badly Going Wrong --- things like writes to the swap device or
> > writes by the page cleaner must succeed, or else Things Would Go Bad
> > In A Hurry).
> 
> After the LUKS data corruption issue was reported I decided to take a
> look at the dm-crypt code. In that code I found the following:
> 
> static void clone_init(struct dm_crypt_io *io, struct bio *clone)
> {
> 	struct crypt_config *cc = io->cc;
> 
> 	clone->bi_private = io;
> 	clone->bi_end_io  = crypt_endio;
> 	bio_set_dev(clone, cc->dev->bdev);
> 	clone->bi_opf	  = io->base_bio->bi_opf;
> }
> [ ... ]
> static struct bio *crypt_alloc_buffer(struct dm_crypt_io *io, unsigned size)
> {
> 	[ ... ]
> 	clone = bio_alloc_bioset(GFP_NOIO, nr_iovecs, &cc->bs);
> 	[ ... ]
> 	clone_init(io, clone);
> 	[ ... ]
> 	for (i = 0; i < nr_iovecs; i++) {
> 		[ ... ]
> 		bio_add_page(clone, page, len, 0);
> 
> 		remaining_size -= len;
> 	}
> 	[ ... ]
> }
> 
> My interpretation is that crypt_alloc_buffer() allocates a bio,
> associates it with the underlying device and clones a bio. The input bio
> may have a size up to UINT_MAX while the new limit for the size of the
> cloned bio is max_sectors * 512. That causes bio_add_page() to fail if
> the input bio is larger than max_sectors * 512, hence the data
> corruption. Please note that this is a guess only and that I'm not
> familiar with the dm-crypt code.
> 
> Bart.

We already had problems with too large bios in dm-crypt and we fixed it by 
adding this piece of code:

        /*
         * Check if bio is too large, split as needed.
         */
        if (unlikely(bio->bi_iter.bi_size > (BIO_MAX_VECS << PAGE_SHIFT)) &&
            (bio_data_dir(bio) == WRITE || cc->on_disk_tag_size))
                dm_accept_partial_bio(bio, ((BIO_MAX_VECS << PAGE_SHIFT) >> SECTOR_SHIFT));

It will ask the device mapper to split the bio if it is too large. So, 
crypt_alloc_buffer can't receive a bio that is larger than BIO_MAX_VECS << 
PAGE_SHIFT.

Mikulas

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