Hi again, I think I have sort of identified the problem. It appears to me that both the block layer and dm-crypt is defected on handling this. First of all, check out the "fallback" zero out implementation, which is used in this case, here: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/block/blk-lib.c?h=v4.12#n309 >From the outer loop, it seem to imply that this should be done in multiple "bio"s, if the request (original "nr_sects") is larger than BIO_MAX_PAGES: ... while (nr_sects != 0) { bio = next_bio(bio, min(nr_sects, (sector_t)BIO_MAX_PAGES), gfp_mask); ... } ... However, there is a inner loop: ... while (nr_sects != 0) { sz = min((sector_t) PAGE_SIZE >> 9 , nr_sects); bi_size = bio_add_page(bio, ZERO_PAGE(0), sz << 9, 0); nr_sects -= bi_size >> 9; sector += bi_size >> 9; if (bi_size < (sz << 9)) break; } ... which apparently would loop over the whole request on its own, making the outer loop a bogus one. The request ends up being done in a single (huge) bio. When the bio is passed on to dm-crypt, it appears that dm-crypt will not split the bio either when it allocates buffer for conversion/encryption: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/md/dm-crypt.c?h=v4.12#n1694 which leads to possible enormous uptake of memory, causing OOM / kernel panic. There seems to be some measure that is suppose to split large bio though: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/md/dm-crypt.c?h=v4.12#n2787 Apparently it is called before kcryptd_crypt_write_convert() / crypt_alloc_buffer(). However, I don't really parse dm_accept_partial_bio() (or the comment about it) so I don't really know what it actually does or how it does it. Neither can I see it helps in reality anyway. Here is another test case that shows the problem: https://ptpb.pw/BWWo.png https://ptpb.pw/aRTE.png Regards, Tom Yan From: Tom Yan Sent: Monday, August 7, 2017 9:58 AM To: dm-devel@xxxxxxxxxx Subject: [BUG] BLKZEROOUT on dm-crypt container cause OOM / kernel panic Hi all, When I do the following: cryptsetup open /dev/sdX[Y] rand --type plain --key-file /dev/random; blkdiscard -z /dev/mapper/rand Some OOM killings and a kernel panic occur. Here is a screenshot: https://gitlab.com/cryptsetup/cryptsetup/uploads/207ffdada52f3172f54a014c67159625/DSC_0129.JPGhttps://gitlab.com/cryptsetup/cryptsetup/uploads/207ffdada52f3172f54a014c67159625/DSC_0129.JPG Similar issue is NOT found in doing `cat /dev/zero > /dev/mapper/rand`, or `blkdiscard -z /dev/sdX[Y]` (without opening a dm-crypt on it), on the same hardware and configuration. Note that `blkdiscard -z` does not trigger the BLKDISCARD ioctl but the BLKZEROOUT ioctl. And the devices I have been testing on are USB devices that does NOT support WRITE SAME or UNMAP. Regards, Tom Yan -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
