| Message ID | 20220326034321.2589FC340E8@smtp.kernel.org |
|---|---|
| State | New |
| Headers | show |
| Series | + mm-fix-unexpected-zeroed-page-mapping-with-zram-swap.patch added to -mm tree | expand |
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.
--- a/mm/page_io.c~mm-fix-unexpected-zeroed-page-mapping-with-zram-swap +++ a/mm/page_io.c @@ -51,54 +51,6 @@ void end_swap_bio_write(struct bio *bio) bio_put(bio); } -static void swap_slot_free_notify(struct page *page) -{ - struct swap_info_struct *sis; - struct gendisk *disk; - swp_entry_t entry; - - /* - * There is no guarantee that the page is in swap cache - the software - * suspend code (at least) uses end_swap_bio_read() against a non- - * swapcache page. So we must check PG_swapcache before proceeding with - * this optimization. - */ - if (unlikely(!PageSwapCache(page))) - return; - - sis = page_swap_info(page); - if (data_race(!(sis->flags & SWP_BLKDEV))) - return; - - /* - * The swap subsystem performs lazy swap slot freeing, - * expecting that the page will be swapped out again. - * So we can avoid an unnecessary write if the page - * isn't redirtied. - * This is good for real swap storage because we can - * reduce unnecessary I/O and enhance wear-leveling - * if an SSD is used as the as swap device. - * But if in-memory swap device (eg zram) is used, - * this causes a duplicated copy between uncompressed - * data in VM-owned memory and compressed data in - * zram-owned memory. So let's free zram-owned memory - * and make the VM-owned decompressed page *dirty*, - * so the page should be swapped out somewhere again if - * we again wish to reclaim it. - */ - disk = sis->bdev->bd_disk; - entry.val = page_private(page); - if (disk->fops->swap_slot_free_notify && __swap_count(entry) == 1) { - unsigned long offset; - - offset = swp_offset(entry); - - SetPageDirty(page); - disk->fops->swap_slot_free_notify(sis->bdev, - offset); - } -} - static void end_swap_bio_read(struct bio *bio) { struct page *page = bio_first_page_all(bio); @@ -114,7 +66,6 @@ static void end_swap_bio_read(struct bio } SetPageUptodate(page); - swap_slot_free_notify(page); out: unlock_page(page); WRITE_ONCE(bio->bi_private, NULL); @@ -394,11 +345,6 @@ int swap_readpage(struct page *page, boo if (sis->flags & SWP_SYNCHRONOUS_IO) { ret = bdev_read_page(sis->bdev, swap_page_sector(page), page); if (!ret) { - if (trylock_page(page)) { - swap_slot_free_notify(page); - unlock_page(page); - } - count_vm_event(PSWPIN); goto out; }
The patch titled Subject: mm: fix unexpected zeroed page mapping with zram swap has been added to the -mm tree. Its filename is mm-fix-unexpected-zeroed-page-mapping-with-zram-swap.patch This patch should soon appear at https://ozlabs.org/~akpm/mmots/broken-out/mm-fix-unexpected-zeroed-page-mapping-with-zram-swap.patch and later at https://ozlabs.org/~akpm/mmotm/broken-out/mm-fix-unexpected-zeroed-page-mapping-with-zram-swap.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next and is updated there every 3-4 working days ------------------------------------------------------ From: Minchan Kim <minchan@kernel.org> Subject: mm: fix unexpected zeroed page mapping with zram swap 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. Link: https://lkml.kernel.org/r/YjTVVxIAsnKAXjTd@google.com Fixes: 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous device") Reported-by: Ivan Babrou <ivan@cloudflare.com> Tested-by: Ivan Babrou <ivan@cloudflare.com> Signed-off-by: Minchan Kim <minchan@kernel.org> Cc: Nitin Gupta <ngupta@vflare.org> Cc: Sergey Senozhatsky <senozhatsky@chromium.org> Cc: Jens Axboe <axboe@kernel.dk> Cc: David Hildenbrand <david@redhat.com> Cc: <stable@vger.kernel.org> [4.14+] Signed-off-by: Andrew Morton <akpm@linux-foundation.org> --- mm/page_io.c | 54 ------------------------------------------------- 1 file changed, 54 deletions(-)