Re: [PATCH v3 07/11] mm: do not split a folio if it has minimum folio order requirement

[Pankaj Raghav <kernel@pankajraghav.com>]

On 26/03/2024 17:23, Zi Yan wrote:
> On 26 Mar 2024, at 12:10, Pankaj Raghav (Samsung) wrote:
> 
>> On Mon, Mar 25, 2024 at 07:06:04PM +0000, Matthew Wilcox wrote:
>>> On Wed, Mar 13, 2024 at 06:02:49PM +0100, Pankaj Raghav (Samsung) wrote:
>>>> From: Pankaj Raghav <p.raghav@samsung.com>
>>>>
>>>> As we don't have a way to split a folio to a any given lower folio
>>>> order yet, avoid splitting the folio in split_huge_page_to_list() if it
>>>> has a minimum folio order requirement.
>>>
>>> FYI, Zi Yan's patch to do that is now in Andrew's tree.
>>> c010d47f107f609b9f4d6a103b6dfc53889049e9 in current linux-next (dated
>>> Feb 26)
>>
>> Yes, I started playing with the patches but I am getting a race condition
>> resulting in a null-ptr-deref for which I don't have a good answer for
>> yet.
>>
>> @zi yan Did you encounter any issue like this when you were testing?
>>
>> I did the following change (just a prototype) instead of this patch:
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 9859aa4f7553..63ee7b6ed03d 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -3041,6 +3041,10 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>  {
>>         struct folio *folio = page_folio(page);
>>         struct deferred_split *ds_queue = get_deferred_split_queue(folio);
>> +       unsigned int mapping_min_order = mapping_min_folio_order(folio->mapping);
> 
> I am not sure if this is right. Since folio can be anonymous and folio->mapping
> does not point to struct address_space.
> 
>> +
>> +       if (!folio_test_anon(folio))

Hmm, but I update the new_order only if it is not anonymous. Do you think it is still
wrong?

>> +               new_order = max_t(unsigned int, mapping_min_order, new_order);
>>         /* reset xarray order to new order after split */
>>         XA_STATE_ORDER(xas, &folio->mapping->i_pages, folio->index, new_order);
>>         struct anon_vma *anon_vma = NULL;
>> @@ -3117,6 +3121,8 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>                         goto out;
>>                 }
>>
>> +               // XXX: Remove it later
>> +               VM_WARN_ON_FOLIO((new_order < mapping_min_order), folio);
>>                 gfp = current_gfp_context(mapping_gfp_mask(mapping) &
>>                                                         GFP_RECLAIM_MASK);
>>
>> I am getting a random null-ptr deref when I run generic/176 multiple
>> times with different blksizes. I still don't have a minimal reproducer
>> for this issue. Race condition during writeback:
>>
>> filemap_get_folios_tag+0x171/0x5c0:
>> arch_atomic_read at arch/x86/include/asm/atomic.h:23
>> (inlined by) raw_atomic_read at include/linux/atomic/atomic-arch-fallback.h:457
>> (inlined by) raw_atomic_fetch_add_unless at include/linux/atomic/atomic-arch-fallback.h:2426
>> (inlined by) raw_atomic_add_unless at include/linux/atomic/atomic-arch-fallback.h:2456
>> (inlined by) atomic_add_unless at include/linux/atomic/atomic-instrumented.h:1518
>> (inlined by) page_ref_add_unless at include/linux/page_ref.h:238
>> (inlined by) folio_ref_add_unless at include/linux/page_ref.h:247
>> (inlined by) folio_ref_try_add_rcu at include/linux/page_ref.h:280
>> (inlined by) folio_try_get_rcu at include/linux/page_ref.h:313
>> (inlined by) find_get_entry at mm/filemap.c:1984
>> (inlined by) filemap_get_folios_tag at mm/filemap.c:2222
>>
>>
>>
>> [  537.863105] ==================================================================
>> [  537.863968] BUG: KASAN: null-ptr-deref in filemap_get_folios_tag+0x171/0x5c0
>> [  537.864581] Write of size 4 at addr 0000000000000036 by task kworker/u32:5/366
>> [  537.865123]
>> [  537.865293] CPU: 6 PID: 366 Comm: kworker/u32:5 Not tainted 6.8.0-11739-g7d0c6e7b5a7d-dirty #795
>> [  537.867201] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
>> [  537.868444] Workqueue: writeback wb_workfn (flush-254:32)
>> [  537.869055] Call Trace:
>> [  537.869341]  <TASK>
>> [  537.869569]  dump_stack_lvl+0x4f/0x70
>> [  537.869938]  kasan_report+0xbd/0xf0
>> [  537.870293]  ? filemap_get_folios_tag+0x171/0x5c0
>> [  537.870767]  ? filemap_get_folios_tag+0x171/0x5c0
>> [  537.871578]  kasan_check_range+0x101/0x1b0
>> [  537.871893]  filemap_get_folios_tag+0x171/0x5c0
>> [  537.872269]  ? __pfx_filemap_get_folios_tag+0x10/0x10
>> [  537.872857]  ? __pfx___submit_bio+0x10/0x10
>> [  537.873326]  ? mlock_drain_local+0x234/0x3f0
>> [  537.873938]  writeback_iter+0x59a/0xe00
>> [  537.874477]  ? __pfx_iomap_do_writepage+0x10/0x10
>> [  537.874969]  write_cache_pages+0x7f/0x100
>> [  537.875396]  ? __pfx_write_cache_pages+0x10/0x10
>> [  537.875892]  ? do_raw_spin_lock+0x12d/0x270
>> [  537.876345]  ? __pfx_do_raw_spin_lock+0x10/0x10
>> [  537.876804]  iomap_writepages+0x88/0xf0
>> [  537.877186]  xfs_vm_writepages+0x120/0x190
>> [  537.877705]  ? __pfx_xfs_vm_writepages+0x10/0x10
>> [  537.878161]  ? lock_release+0x36f/0x670
>> [  537.878521]  ? __wb_calc_thresh+0xe5/0x3b0
>> [  537.878892]  ? __pfx_lock_release+0x10/0x10
>> [  537.879308]  do_writepages+0x170/0x7a0
>> [  537.879676]  ? __pfx_do_writepages+0x10/0x10
>> [  537.880182]  ? writeback_sb_inodes+0x312/0xe40
>> [  537.880689]  ? reacquire_held_locks+0x1f1/0x4a0
>> [  537.881193]  ? writeback_sb_inodes+0x312/0xe40
>> [  537.881665]  ? find_held_lock+0x2d/0x110
>> [  537.882104]  ? lock_release+0x36f/0x670
>> [  537.883344]  ? wbc_attach_and_unlock_inode+0x3b8/0x710
>> [  537.883853]  ? __pfx_lock_release+0x10/0x10
>> [  537.884229]  ? __pfx_lock_release+0x10/0x10
>> [  537.884604]  ? lock_acquire+0x138/0x2f0
>> [  537.884952]  __writeback_single_inode+0xd4/0xa60
>> [  537.885369]  writeback_sb_inodes+0x4cf/0xe40
>> [  537.885760]  ? __pfx_writeback_sb_inodes+0x10/0x10
>> [  537.886208]  ? __pfx_move_expired_inodes+0x10/0x10
>> [  537.886640]  __writeback_inodes_wb+0xb4/0x200
>> [  537.887037]  wb_writeback+0x55b/0x7c0
>> [  537.887372]  ? __pfx_wb_writeback+0x10/0x10
>> [  537.887750]  ? lock_acquire+0x138/0x2f0
>> [  537.888094]  ? __pfx_register_lock_class+0x10/0x10
>> [  537.888521]  wb_workfn+0x648/0xbb0
>> [  537.888835]  ? __pfx_wb_workfn+0x10/0x10
>> [  537.889192]  ? lock_acquire+0x128/0x2f0
>> [  537.889539]  ? lock_acquire+0x138/0x2f0
>> [  537.889890]  process_one_work+0x7ff/0x1710
>> [  537.890272]  ? __pfx_process_one_work+0x10/0x10
>> [  537.890685]  ? assign_work+0x16c/0x240
>> [  537.891026]  worker_thread+0x6e8/0x12b0
>> [  537.891381]  ? __pfx_worker_thread+0x10/0x10
>> [  537.891768]  kthread+0x2ad/0x380
>> [  537.892064]  ? __pfx_kthread+0x10/0x10
>> [  537.892403]  ret_from_fork+0x2d/0x70
>> [  537.892728]  ? __pfx_kthread+0x10/0x10
>> [  537.893068]  ret_from_fork_asm+0x1a/0x30
>> [  537.893434]  </TASK>
> 
> 
> --
> Best Regards,
> Yan, Zi