On Tue, 15 Oct 2013, Krzysztof Kozlowski wrote: > On Tue, 2013-10-15 at 02:59 -0700, Hugh Dickins wrote: > > On Mon, 14 Oct 2013, Krzysztof Kozlowski wrote: > > > > > Fix race between swapoff and swapon resulting in setting blocksize of > > > PAGE_SIZE for block devices during swapoff. > > > > > > The swapon modifies swap_info->old_block_size before acquiring > > > swapon_mutex. It reads block_size of bdev, stores it under > > > swap_info->old_block_size and sets new block_size to PAGE_SIZE. > > > > > > On the other hand the swapoff sets the device's block_size to > > > old_block_size after releasing swapon_mutex. > > > > > > This patch locks the swapon_mutex much earlier during swapon. It also > > > releases the swapon_mutex later during swapoff. > > > > > > The effect of race can be triggered by following scenario: > > > - One block swap device with block size of 512 > > > - thread 1: Swapon is called, swap is activated, > > > p->old_block_size = block_size(p->bdev); /512/ > > > block_size(p->bdev) = PAGE_SIZE; > > > Thread ends. > > > > > > - thread 2: Swapoff is called and it goes just after releasing the > > > swapon_mutex. The swap is now fully disabled except of setting the > > > block size to old value. The p->bdev->block_size is still equal to > > > PAGE_SIZE. > > > > > > - thread 3: New swapon is called. This swap is disabled so without > > > acquiring the swapon_mutex: > > > - p->old_block_size = block_size(p->bdev); /PAGE_SIZE (!!!)/ > > > - block_size(p->bdev) = PAGE_SIZE; > > > Swap is activated and thread ends. > > > > > > - thread 2: resumes work and sets blocksize to old value: > > > - set_blocksize(bdev, p->old_block_size) > > > But now the p->old_block_size is equal to PAGE_SIZE. > > > > > > The patch swap-fix-set_blocksize-race-during-swapon-swapoff does not fix > > > this particular issue. It reduces the possibility of races as the swapon > > > must overwrite p->old_block_size before acquiring swapon_mutex in > > > swapoff. > > > > > > Signed-off-by: Krzysztof Kozlowski <k.kozlowski@xxxxxxxxxxx> > > > > Sorry you're being blown back and forth on this, but I say Nack to > > this version. I've not spent the time to check whether it ends up > > correct or not; but your original patch was appropriate to the bug, > > and this one is just unnecessary churn in my view. > > Hi, > > I still think my previous patch does not solve the issue entirely. > The call set_blocksize() in swapoff quite often sets PAGE_SIZE instead > of valid block size (e.g. 512). I trigger this with: PAGE_SIZE and 512 are equally valid block sizes, it's just hard to support both consistently at the same instant. > ------ > for i in `seq 1000` > do > swapoff /dev/sdc1 & > swapon /dev/sdc1 & > swapon /dev/sdc1 & > done > ------ > 10 seconds run of this script resulted in 50% of set_blocksize(PAGE_SIZE). > Although effect can only be observed after adding printks (block device is > released). But despite PAGE_SIZE being a valid block size, I agree that it's odd if you see variation there. Here's my guess: it looks as if the p->bdev test is inadequate, in the decision whether bad_swap should set_blocksize() or not: p->bdev is not usually reset when a swap_info_struct is released for reuse. Please try correcting that, either by resetting p->bdev where necessary, or by putting a better test in bad_swap: see if that fixes this oddity. I still much prefer your original little patch, to this extension of the use of swapon_mutex. However, a bigger question would be, why does swapoff have to set block size back to old_block_size anyway? That was introduced in 2.5.13 by <viro@xxxxxxxxxxxx> (02/05/01 1.447.69.1) [PATCH] (1/6) blksize_size[] removal - preliminary cleanups: make sure that swapoff restores original block size, kill set_blocksize() (and use of __bread()) in multipath.c, reorder opening device and finding its block size in mtdblock.c. Al, not an urgent question, but is this swapoff old_block_size stuff still necessary? And can't swapon just use whatever bd_block_size is already in force? IIUC, it plays no part beyond the initial readpage of swap header. Thanks, Hugh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>