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

[Adding Jan Kara; retaining whole message for context]

On Mon, Jun 29, 2026 at 09:39:25AM +0800, Chao Yu wrote:
> 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?

In general, yes, memalloc_nofs_save() after locking dqio_mutex sounds like
the right idea to me.  I'd want to familiarise myself more thoroughly
with the code before making a firm recommendation, and it's probably
quicker to just ask Jan ;-)


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