Re: [f2fs-dev] [PATCH] generic/064: allow 50 extents on F2FS after fcollapse

Hi Zorro,

On 7/19/2026 5:24 PM, Zorro Lang wrote:
> On Tue, Jun 30, 2026 at 08:09:19PM +0800, Chao Yu wrote:
>> On 6/30/26 00:37, Zorro Lang via Linux-f2fs-devel wrote:
>>> On Wed, Jun 24, 2026 at 12:46:01AM -0700, Christoph Hellwig wrote:
>>>> This sounds odd.  The test allocates a contigous range and then just does
>>>> insert/collapse on it, which should not lead to any new data block
>>>> allocations.  Given that the test works fine on zoned XFS and btrfs
>>>> with strict out of place write policies we know it does not require
>>>> overwriting blocks to work as well.
>>>>
>>>> So I think something is fishy in f2fs if needs to allocate data blocks
>>>> here.
>>>
>>> I have the same question with Christoph. Shouldn't fcollapse in F2FS just
>>> remap the metadata instead of allocating new data blocks?
>>
>> Hi Christoph, Zorro,
>>
>> Let me explain it briefly, this is because f2fs has only one copy of reverse
>> mapping table (aka SSA), for any checkpointed data, we can not modify SSA info
>> directly during remap, otherwise, after sudden power-cut, we will suffer
>> inconsistent issue in between reverse mapping table and mapping table. So, f2fs
>> choose to rewrite for any checkpointed data.
> 
> Hi Chao,
> 
> Thank you for the detailed explanation. I now see why F2FS handles this
> differently.
> 
> Since this is by design for F2FS, I'd like to merge this fix to help f2fs
> avoid this test failure at first. As for the potential write amplification
> and performance overhead introduced by this rewrite strategy, that can be
> discussed further on the f2fs list if necessary.

It makes sense to me, we can discuss about the detailed implementation in f2fs
list if there is any concern.

> 
> I will merge the patch with some clarifying comments:
> 
> /*
>   * F2FS uses a rewrite strategy for fcollapse/finsert on checkpointed data.
>   * Since F2FS has only a single copy of the reverse mapping table (SSA),
>   * directly remapping checkpointed metadata could cause inconsistency
>   * issues after a sudden power-cut.
>   */
> 
> Is this ^^ good to you, or feel free to give me a proper comment to add into
> this patch.

Looks good, thanks for the help!

Thanks

> 
> Thanks,
> Zorro
> 
>>
>> Thanks,