Re: + mm-fix-unexpected-zeroed-page-mapping-with-zram-swap.patch added to -mm tree

[Sergey Senozhatsky <senozhatsky@chromium.org>]

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?