Re: [f2fs-dev] Use of FGP_NOFS in f2fs

[Chao Yu via Linux-f2fs-devel <linux-f2fs-devel@lists.sourceforge.net>]

On 6/26/26 17:30, Jiucheng Xu wrote:
> 
> 
> On 6/25/2026 3:17 AM, Matthew Wilcox wrote:
>> [ EXTERNAL EMAIL ]
>>
>> Hi Jiucheng,
>>
>> I am trying to remove FGP_NOFS from the kernel and the last remaining
>> user was added by you last year in commit 2308de27c03d.  I'm trying to
>> understand why.  Did you see an actual problem if you do not use it,
>> or was it theoretical?  The commit message says "to avoid potential
>> deadlock issues", but it's not clear to me whether you know they are
>> there, or only think they are there.
>>
>> I'd really like to understand what the issues are as the normal issues
>> which make calling into the filesystem (holding another folio locked,
>> holding a mutex relied upon by reclaim) don't seem to be present.
>> So if there is something, I'm not sure what it is.
> Hi Matthew,
> 
> The FGP_NOFS flag was suggested by Chao and not adding it might lead to deadlock issues.
> 
> @Chao could you explain this for Matthew?

Hi Matthew, Jiucheng,

IIRC, for normal path from write -> write_begin, it seems fine since there is
no f2fs-specified mutex or other folio lock in the path, but I was worried about
quota path when I suggested to keep GFP_NOFS flag, as the lock race condition
is quite complicated there: f2fs internal lock vs quota system lock vs folio lock...

I searched the commits and found this:

Commit 02117b8ae9c0 ("f2fs: Set GF_NOFS in read_cache_page_gfp while doing
f2fs_quota_read")

In this case, f2fs_quota_read() was calling read_mapping_page(), which allowed
GFP_FS allocations. This triggered the following deadlock:

     Thread 1 (User Open/Write)
     - do_sys_open
      - vfs_open
       - dquot_file_open
        - dquot_initialize
         - dqget
          - dquot_acquire
           : locks &dqopt->dqio_mutex (VFS Quota Mutex)
           - qtree_read_dquot
            - f2fs_quota_read
             - read_mapping_page (GFP_KERNEL / allows GFP_FS)
              - __alloc_pages_nodemask
               - try_to_free_pages (Direct Reclaim)
                - prune_icache_sb
                 - evict
                  - f2fs_evict_inode
                   - dquot_drop
                    - dqput
                     - dquot_commit
                      : tries to lock &dqopt->dqio_mutex again
                      ==> DEADLOCK (waiting for itself)

It may trigger deadlock in f2fs_quota_write() as the same way in f2fs_quota_read()
path? let me know if I'm missing anything.

Are memalloc_nofs_save() and memalloc_nofs_restore() recommended for such case?
any suggestions?

Thanks,


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