Re: [PATCH] ntfs: wait for bitmap folio writeback before modifying it

[Hyunchul Lee]

Hi Shengzhuo,

On Thu, Oct 29, 2026 at 02:17:56PM +0800, Shengzhuo Wei wrote:
> On 2026-10-29 10:07, liubaolin wrote:
>  
> > Hi Shengzhuo,
> >    Could this deadlock during MFT bitmap
> > expansion？ntfs_mft_bitmap_extend_allocation_nolock() can recurse into MFT
> > record allocation while holding mftbmp_ni->runlist.lock.
> > 
> >   With L being that lock, could we get the following interleaving?
> > A and B are $MFT::$BITMAP folios; B is outside the cached iomap.
> > Time flows downwards.
> > 
> >   Writeback                        Bitmap expansion
> >   ---------                        ----------------
> >   lock A
> >   set PG_writeback(A)
> >   map A (take/release L)
> >   add A to wpc->wb_ctx
> >   unlock A
> >   [BIO still unsubmitted]
> >                                    take L
> >                                    extend bitmap mapping pairs
> >                                    need another extent record
> >                                    ntfs_mft_record_alloc()
> >                                      mft_bitmap_alloc_free_rec()
> >                                        lock A
> >                                        folio_wait_writeback(A)
> >                                    [wait for A's I/O, holding L]
> > 
> >   lock B
> >   set PG_writeback(B)
> >   ntfs_writeback_range(B)
> >     __ntfs_write_iomap_begin()
> >       down_write(L)
> >       [blocked on L]
> > 
> >   Cannot reach iomap_add_to_ioend()
> >   to submit A's pending BIO.        Cannot release L until
> >                                    A's writeback completes.
> > 
> >    ntfs_writeback_range() obtains the next mapping before
> > iomap_add_to_ioend() can submit the pending BIO.
> >    Would we need to submit that BIO before taking the mapping locks?
> > 
> > Thanks,
> > Baolin.
> > 
> 
> Hi Baolin,
> 
> Thanks for the analysis. I checked the paths and agree with you.
> 
> Following your suggestion, I prepared a change to submit the pending
> ioend before obtaining a new mapping in ntfs_writeback_range(), clearing
> wpc->wb_ctx even on error to avoid resubmission. I plan to make this a
> prerequisite for v2, retaining all six waits.
> 
> One concern is that submitting at mapping boundaries may reduce BIO
> aggregation, including for ordinary file writeback. The change has
> passed object builds, but has not been runtime- or performance-tested.
> 
> Does this approach look reasonable to you?

I think the early submit should be limited to $MFT:$BITMAP only.

> 
> Thanks,
> Shengzhuo

-- 
Thanks,
Hyunchul