On Wed, Oct 16, 2013 at 1:19 AM, Hugh Dickins <hu...@google.com> wrote: > 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.kozlow...@samsung.com> >> > >> > 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 > > <v...@math.psu.edu> (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
Let me try to explain(and guess): we have to set_block in swapon. the swap_header is PAGE_SIZE, if device's blocksize is more than PAGE_SIZE, then the swap entry address on swapfile would be not PAGE_SIZE aligned. or one swap page can not fill a block. There maybe a problem for some device. The set_blocksize() do the judgement work for swapon. And may be some userland tools assume swap device blocksize is PAGE_SIZE？ issues here are more than this one: After swap_info_struct is released for reuse in swapoff. Its corresponding resources are released later, such as: - swap_cgroup_swapoff(type); - blkdev_put - inode->i_flags &= ~S_SWAPFILE; we need release(or clean) these resources before release swap_info_struct. to Krzysztof: I think it is better to add this handle to your patch regards -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
