Re: [PATCH RFC] xarray: honor XA_FLAGS_ACCOUNT in xas_split_alloc()

[Zi Yan <ziy@nvidia.com>]

On 30 Jul 2026, at 10:13, Johannes Weiner wrote:

> Hello Zi,
>
> On Mon, Jul 27, 2026 at 09:51:40PM -0400, Zi Yan wrote:
>> XArray operations that allocate xa_nodes, such as xas_nomem() and
>> xas_alloc(), add __GFP_ACCOUNT when the array has XA_FLAGS_ACCOUNT set.
>> This charges the allocated memory and avoids the workingset convergence
>> issue described by commit 7b785645e8f13 ("mm: fix page cache convergence
>> regression").
>>
>> xas_split_alloc() does not have that flag. Add it when necessary.
>>
>> Fixes: 6b24ca4a1a8d4 ("mm: Use multi-index entries in the page cache")
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> ---
>> Hi Johannes,
>>
>> IIUC, __GFP_ACCOUNT is needed for xarray node allocation accounting when
>> XA_FLAGS_ACCOUNT is set. Commit 7b785645e8f13 ("mm: fix page cache
>> convergence regression") fixed a workingset regression with it.
>> xas_split_alloc() does not have it, so I imagine xa_node allocated during
>> folio split would cause a similar issue. I would like to get your
>> opinion on this.
>
> Yes, you're right!
>
> As we had discussed on the THP cabal call, we should use the memcg
> context of the folio, as that could be different from the callers'
> depending on who's doing the splitting. I.e.
>
> 	memcg = get_mem_cgroup_from_folio(x);
> 	old_memcg = set_active_memcg(memcg);
>
> 	xas_split_alloc() / xas_try_split()
>
> 	set_active_memcg(old_memcg);
> 	mem_cgroup_put(memcg);
>
> There is __folio_split() -> xas_split_alloc(). But there is also
> __folio_split() -> __folio_freeze_and_split_unmapped() ->
> __split_unmapped_folio() -> xas_try_split() -> XA_FLAGS_ACCOUNT ->
> __GFP_ACCOUNT. So it would make sense to me to set up the memcg
> context in __folio_split() already.

Thank you for the detailed analysis. Will add memcg context code
and send a new version.


Best Regards,
Yan, Zi