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

["Shengzhuo Wei" <me@cherr.cc>]

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?

Thanks,
Shengzhuo