Re: [PATCH v2] btrfs: fix the delalloc range locking if sector size < page size

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2024/10/9 03:28, David Sterba 写道:
> On Mon, Oct 07, 2024 at 09:51:40AM +1030, Qu Wenruo wrote:
>> Inside lock_delalloc_folios(), there are several problems related to
>> sector size < page size handling:
>>
>> - Set the writer locks without checking if the folio is still valid
>>    We call btrfs_folio_start_writer_lock() just like it's folio_lock().
>>    But since the folio may not even be the folio of the current mapping,
>>    we can easily screw up the folio->private.
>>
>> - The range is not clampped inside the page
>>    This means we can over write other bitmaps if the start/len is not
>>    properly handled, and trigger the btrfs_subpage_assert().
>>
>> - @processed_end is always rounded up to page end
>>    If the delalloc range is not page aligned, and we need to retry
>>    (returning -EAGAIN), then we will unlock to the page end.
>>
>>    Thankfully this is not a huge problem, as now
>>    btrfs_folio_end_writer_lock() can handle range larger than the locked
>>    range, and only unlock what is already locked.
>>
>> Fix all these problems by:
>>
>> - Lock and check the folio first, then call
>>    btrfs_folio_set_writer_lock()
>>    So that if we got a folio not belonging to the inode, we won't
>>    touch folio->private.
>>
>> - Properly truncate the range inside the page
>>
>> - Update @processed_end to the locked range end
>>
>> - Remove btrfs_folio_start_writer_lock()
>>    Since all callsites only utilize btrfs_folio_set_writer_lock() now,
>>    remove the unnecessary btrfs_folio_start_writer_lock().
>>
>> Fixes: 1e1de38792e0 ("btrfs: make process_one_page() to handle subpage locking")
>
> This commit is in 5.14, so we'd need backport to 5.15+. The v1 would be
> more suitable as it does not remove the unused
> btrfs_folio_start_writer_lock(). As it's exported function there should
> be no warning.
>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
>> Changelog:
>> v2:
>> - Remove the unused btrfs_folio_start_writer_lock() function
>> ---
>>   fs/btrfs/extent_io.c | 15 +++++++-------
>>   fs/btrfs/subpage.c   | 47 --------------------------------------------
>>   fs/btrfs/subpage.h   |  2 --
>>   3 files changed, 7 insertions(+), 57 deletions(-)
>>
>> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
>> index e0b43640849e..0448cee2b983 100644
>> --- a/fs/btrfs/extent_io.c
>> +++ b/fs/btrfs/extent_io.c
>> @@ -262,22 +262,21 @@ static noinline int lock_delalloc_folios(struct inode *inode,
>>
>>   		for (i = 0; i < found_folios; i++) {
>>   			struct folio *folio = fbatch.folios[i];
>> -			u32 len = end + 1 - start;
>> +			u64 range_start = max_t(u64, folio_pos(folio), start);
>> +			u32 range_len = min_t(u64, folio_pos(folio) + folio_size(folio),
>> +					      end + 1) - range_start;
>>
>>   			if (folio == locked_folio)
>>   				continue;
>>
>> -			if (btrfs_folio_start_writer_lock(fs_info, folio, start,
>> -							  len))
>> -				goto out;
>> -
>> +			folio_lock(folio);
>>   			if (!folio_test_dirty(folio) || folio->mapping != mapping) {
>
> Should the range_start and range_end be set after folio_lock? One of the
> prolblems you're fixing is the folio and mapping mismatch, if I read it
> correctly.
>
Nice catch, indeed that's the problem.

Will fix it and split out the cleanup in the next version.

Thanks,
Qu