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

From: Pankaj Raghav (Samsung)
Date: Tue Mar 26 2024 - 12:11:05 EST

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@xxxxxxxxxxx>
> >
> > 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);
+
+ if (!folio_test_anon(folio))
+ 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>