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Date: Thu, 25 Aug 2022 09:41:03 +0800
From: Chao Yu <chao@...nel.org>
To: Jaegeuk Kim <jaegeuk@...nel.org>
Cc: linux-f2fs-devel@...ts.sourceforge.net,
linux-kernel@...r.kernel.org, Shuqi Zhang <zhangshuqi3@...wei.com>
Subject: Re: [f2fs-dev] [PATCH -next] f2fs: fix wrong dirty page count when
race between mmap and fallocate.
On 2022/8/21 6:19, Jaegeuk Kim wrote:
> On 08/20, Jaegeuk Kim wrote:
>> On 08/20, Chao Yu wrote:
>>> On 2022/8/15 16:15, Shuqi Zhang wrote:
>>>> This is a BUG_ON issue as follows when running xfstest-generic-503:
>>>> WARNING: CPU: 21 PID: 1385 at fs/f2fs/inode.c:762 f2fs_evict_inode+0x847/0xaa0
>>>> Modules linked in:
>>>> CPU: 21 PID: 1385 Comm: umount Not tainted 5.19.0-rc5+ #73
>>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-4.fc34 04/01/2014
>>>>
>>>> Call Trace:
>>>> evict+0x129/0x2d0
>>>> dispose_list+0x4f/0xb0
>>>> evict_inodes+0x204/0x230
>>>> generic_shutdown_super+0x5b/0x1e0
>>>> kill_block_super+0x29/0x80
>>>> kill_f2fs_super+0xe6/0x140
>>>> deactivate_locked_super+0x44/0xc0
>>>> deactivate_super+0x79/0x90
>>>> cleanup_mnt+0x114/0x1a0
>>>> __cleanup_mnt+0x16/0x20
>>>> task_work_run+0x98/0x100
>>>> exit_to_user_mode_prepare+0x3d0/0x3e0
>>>> syscall_exit_to_user_mode+0x12/0x30
>>>> do_syscall_64+0x42/0x80
>>>> entry_SYSCALL_64_after_hwframe+0x46/0xb0
>>>>
>>>> Function flow analysis when BUG occurs:
>>>> f2fs_fallocate mmap
>>>> do_page_fault
>>>> pte_spinlock // ---lock_pte
>>>> do_wp_page
>>>> wp_page_shared
>>>> pte_unmap_unlock // unlock_pte
>>>> do_page_mkwrite
>>>> f2fs_vm_page_mkwrite
>>>> down_read(i_mmap_sem)
>>>> lock_page
>>>> if (PageMappedToDisk(page))
>>>> goto out;
>>>> // set_page_dirty --NOT RUN
>>>> out: up_read(i_mmap_sem)
>>>> lock_page
>>>> finish_mkwrite_fault // unlock_pte
>>>> f2fs_collapse_range
>>>> down_write(i_mmap_sem)
>>>> truncate_pagecache
>>>> unmap_mapping_pages
>>>> i_mmap_lock_write // down_write(i_mmap_rwsem)
>>>> ......
>>>> zap_pte_range
>>>> pte_offset_map_lock // ---lock_pte
>>>> f2fs_set_data_page_dirty
>>>
>>> I didn't get it, why zap_pte_range() can set page dirty w/o lock_page?
>>>
>>> I found it's very easy to reproduce this bug, but previously I never saw this...
>>> is there any code udpate around truncate_pagecache()?
>>
>> Found this.
>>
>> 2637 * The caller must ensure this doesn't race with truncation. Most will
>> 2638 * simply hold the folio lock, but e.g. zap_pte_range() calls with the
>> 2639 * folio mapped and the pte lock held, which also locks out truncation.
>> 2640 */
>> 2641 bool filemap_dirty_folio(struct address_space *mapping, struct folio *folio)
>> 2642 {
>>
>>>
>>> Thanks,
>>>
>>>> if (!PageDirty(page)) {
>>>> fault_dirty_shared_page
>>>> f2fs_set_data_page_dirty
>>>> if (!PageDirty(page)) {
>>>> __set_page_dirty_nobuffer
>>>> f2fs_update_dirty_page // ++
>>>> }
>>>> unlock_page
>>>> __set_page_dirty_nobuffers
>>>> f2fs_update_dirty_page // page count++
>>>> }
>>>> pte_unmap_unlock // --unlock_pte
>>>> i_mmap_unlock_write // up_write(i_mmap_rwsem)
>>>> truncate_inode_pages
>>>> up_write(i_mmap_sem)
>>>>
>>>> When race happens between mmap-do_page_fault-wp_page_shared and
>>>> fallocate-truncate_pagecache-zap_pte_range, the zap_pte_range calls
>>>> function set_page_dirty without page lock. Besides, though
>>>> truncate_pagecache has immap and pte lock, wp_page_shared calls
>>>> fault_dirty_shared_page without any. In this case, two threads race
>>>> in f2fs_set_data_page_dirty function. Page is set to dirty only ONCE,
>>>> but the count is added TWICE by calling f2fs_update_dirty_page.
>>>> Thus the count of dirty page cannot accord with the real dirty pages.
>>>>
>>>> Following is the solution to in case of race happens without any lock.
>>>> If making sure f2fs_vm_page_mkwrite calls set_page_dirty within immap
>>>> lock area, page will already be dirtied when running into
>>>> fault_dirty_shared_page-f2fs_set_data_page_dirty.
>>>> The count of dirty page will not be increased wrong times.
>>>>
>>>> Signed-off-by: Shuqi Zhang <zhangshuqi3@...wei.com>
>>>> ---
>>>> fs/f2fs/file.c | 11 ++++++-----
>>>> 1 file changed, 6 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
>>>> index ce4905a073b3..d837359a9c00 100644
>>>> --- a/fs/f2fs/file.c
>>>> +++ b/fs/f2fs/file.c
>>>> @@ -140,7 +140,7 @@ static vm_fault_t f2fs_vm_page_mkwrite(struct vm_fault *vmf)
>>>> * check to see if the page is mapped already (no holes)
>>>> */
>>>> if (PageMappedToDisk(page))
>>>> - goto out_sem;
>>>> + goto set_dirty;
>>>> /* page is wholly or partially inside EOF */
>>>> if (((loff_t)(page->index + 1) << PAGE_SHIFT) >
>>>> @@ -150,14 +150,15 @@ static vm_fault_t f2fs_vm_page_mkwrite(struct vm_fault *vmf)
>>>> offset = i_size_read(inode) & ~PAGE_MASK;
>>>> zero_user_segment(page, offset, PAGE_SIZE);
>>>> }
>>>> - set_page_dirty(page);
>>>> - if (!PageUptodate(page))
>>>> - SetPageUptodate(page);
>>>> f2fs_update_iostat(sbi, APP_MAPPED_IO, F2FS_BLKSIZE);
>>>> - f2fs_update_time(sbi, REQ_TIME);
>>>> +set_dirty:
>>>> trace_f2fs_vm_page_mkwrite(page, DATA);
>>>> + set_page_dirty(page);
>>>> + if (!PageUptodate(page))
>>>> + SetPageUptodate(page);
>
> Actually we don't need to call SetPageUptodate() since set_page_dirty() should
> do? And, it seems the call stack is out-dated as well.
>
> By the way, do we just need to get the right count by this?
>
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -3697,8 +3697,7 @@ static bool f2fs_dirty_data_folio(struct address_space *mapping,
> folio_mark_uptodate(folio);
> BUG_ON(folio_test_swapcache(folio));
>
> - if (!folio_test_dirty(folio)) {
> - filemap_dirty_folio(mapping, folio);
> + if (filemap_dirty_folio(mapping, folio)) {
Agreed, how about checking return value of filemap_dirty_folio() in
f2fs_dirty_{meta,node}_folio as well for cleanup.
> f2fs_update_dirty_folio(inode, folio);
> return true;
I suspect we may still suffer from below race condition? due to we will access/update
page private stuff w/o page lock in zap_pte_range.
- zap_pte_range - f2fs_defragment_range
- f2fs_dirty_data_folio
- f2fs_update_dirty_folio - lock_page
- set_page_private_reference - set_page_private_gcing
Thanks,
> }
>
>>>> + f2fs_update_time(sbi, REQ_TIME);
>>>> out_sem:
>>>> filemap_invalidate_unlock_shared(inode->i_mapping);
>>
>>
>> _______________________________________________
>> Linux-f2fs-devel mailing list
>> Linux-f2fs-devel@...ts.sourceforge.net
>> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
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