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SubjectRe: [PATCH v4 4/5] mm/shmem: fix infinite loop when swap in shmem error at swapoff time
Date
On Fri, May 20, 2022 at 04:17:45PM +0800, Miaohe Lin wrote:
> On 2022/5/20 14:34, HORIGUCHI NAOYA(堀口 直也) wrote:
> > On Thu, May 19, 2022 at 08:50:29PM +0800, Miaohe Lin wrote:
> >> When swap in shmem error at swapoff time, there would be a infinite loop
> >> in the while loop in shmem_unuse_inode(). It's because swapin error is
> >> deliberately ignored now and thus info->swapped will never reach 0. So
> >> we can't escape the loop in shmem_unuse().
> >>
> >> In order to fix the issue, swapin_error entry is stored in the mapping
> >> when swapin error occurs. So the swapcache page can be freed and the
> >> user won't end up with a permanently mounted swap because a sector is
> >> bad. If the page is accessed later, the user process will be killed
> >> so that corrupted data is never consumed. On the other hand, if the
> >> page is never accessed, the user won't even notice it.
> >>
> >> Reported-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> >> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> >
> > Hi Miaohe,
> >
> > Thank you for the update. I might miss something, but I still see the same
> > problem (I checked it on mm-everything-2022-05-19-00-03 + this patchset).
>
> I was testing this patch on my 5.10 kernel. I reproduced the problem in my env and
> fixed it. It seems there might be some critical difference though I checked that by
> reviewing the code... Sorry. :(
>
> >
> > This patch has the effect to change the return value of shmem_swapin_folio(),
> > -EIO (without this patch) to -EEXIST (with this patch).
>
> In fact, I didn't change the return value from -EIO to -EEXIST:
>
> @@ -1762,6 +1799,8 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
> failed:
> if (!shmem_confirm_swap(mapping, index, swap))
> error = -EEXIST;
> + if (error == -EIO)
> + shmem_set_folio_swapin_error(inode, index, folio, swap)
>
> > But shmem_unuse_swap_entries() checks neither, so no change from caller's view point.
> > Maybe breaking in errors (rather than ENOMEM) in for loop in shmem_unuse_swap_entries()
> > solves the issue? I briefly checked with the below change, then swapoff can return
> > with failure.
> >
> > @@ -1222,7 +1222,7 @@ static int shmem_unuse_swap_entries(struct inode *inode,
> > folio_put(folio);
> > ret++;
> > }
> > - if (error == -ENOMEM)
> > + if (error < 0)
> > break;
> > error = 0;
> > }
>
> Yes, this is the simplest and straightforward way to fix the issue. But it has the side effect
> that user will end up with a permanently mounted swap just because a sector is bad. That might
> be somewhat unacceptable?

Ah, you're right, swapoff should return with success instead of with
failure. I tried the fix in your another email, and that makes swapoff
return with success, so your fix looks better than mine.

Thanks,
Naoya Horiguchi
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