From: Sergey Senozhatsky <senozhatsky@chromium.org>
To: Minchan Kim <minchan@kernel.org>,
Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
mm-commits@vger.kernel.org, stable@vger.kernel.org,
ngupta@vflare.org, ivan@cloudflare.com, david@redhat.com,
axboe@kernel.dk
Subject: Re: + mm-fix-unexpected-zeroed-page-mapping-with-zram-swap.patch added to -mm tree
Date: Wed, 30 Mar 2022 14:07:21 +0900 [thread overview]
Message-ID: <YkPliXgyWSEd+RvQ@google.com> (raw)
In-Reply-To: <YkPkYmn6E7DY7EPP@google.com>
On (22/03/30 14:02), Sergey Senozhatsky wrote:
> On (22/03/25 20:43), Andrew Morton wrote:
> > Two processes under CLONE_VM cloning, user process can be corrupted by
> > seeing zeroed page unexpectedly.
> >
> > CPU A CPU B
> >
> > do_swap_page do_swap_page
> > SWP_SYNCHRONOUS_IO path SWP_SYNCHRONOUS_IO path
> > swap_readpage valid data
> > swap_slot_free_notify
> > delete zram entry
> > swap_readpage zeroed(invalid) data
> > pte_lock
> > map the *zero data* to userspace
> > pte_unlock
> > pte_lock
> > if (!pte_same)
> > goto out_nomap;
> > pte_unlock
> > return and next refault will
> > read zeroed data
> >
> > The swap_slot_free_notify is bogus for CLONE_VM case since it doesn't
> > increase the refcount of swap slot at copy_mm so it couldn't catch up
> > whether it's safe or not to discard data from backing device. In the
> > case, only the lock it could rely on to synchronize swap slot freeing is
> > page table lock. Thus, this patch gets rid of the swap_slot_free_notify
> > function. With this patch, CPU A will see correct data.
> >
> > CPU A CPU B
> >
> > do_swap_page do_swap_page
> > SWP_SYNCHRONOUS_IO path SWP_SYNCHRONOUS_IO path
> > swap_readpage original data
> > pte_lock
> > map the original data
> > swap_free
> > swap_range_free
> > bd_disk->fops->swap_slot_free_notify
> > swap_readpage read zeroed data
> > pte_unlock
> > pte_lock
> > if (!pte_same)
> > goto out_nomap;
> > pte_unlock
> > return
> > on next refault will see mapped data by CPU B
> >
> > The concern of the patch would increase memory consumption since it could
> > keep wasted memory with compressed form in zram as well as uncompressed
> > form in address space. However, most of cases of zram uses no readahead
> > and do_swap_page is followed by swap_free so it will free the compressed
> > form from in zram quickly.
>
> Minchan, a quick question, shouldn't this instead revert 3f2b1a04f4493?
Never mind! My bad.
The patch looks good to me.
prev parent reply other threads:[~2022-03-30 5:07 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-26 3:43 + mm-fix-unexpected-zeroed-page-mapping-with-zram-swap.patch added to -mm tree Andrew Morton
2022-03-30 5:02 ` Sergey Senozhatsky
2022-03-30 5:07 ` Sergey Senozhatsky [this message]
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