Re: [PATCH v6 1/5] mm/filemap: add mempolicy support to the filemap layer

Ackerley Tng
2025-02-28T17:51:02Z

Vlastimil Babka <vbabka@suse.cz> writes:

> On 2/26/25 09:25, Shivank Garg wrote:
>> From: Shivansh Dhiman <shivansh.dhiman@amd.com>
>>
>> Add NUMA mempolicy support to the filemap allocation path by introducing
>> new APIs that take a mempolicy argument:
>> - filemap_grab_folio_mpol()
>> - filemap_alloc_folio_mpol()
>> - __filemap_get_folio_mpol()
>>
>> These APIs allow callers to specify a NUMA policy during page cache
>> allocations, enabling fine-grained control over memory placement. This is
>> particularly needed by KVM when using guest-memfd memory backends, where
>> the guest memory needs to be allocated according to the NUMA policy
>> specified by VMM.
>>
>> The existing non-mempolicy APIs remain unchanged and continue to use the
>> default allocation behavior.
>>
>> Signed-off-by: Shivansh Dhiman <shivansh.dhiman@amd.com>
>> Signed-off-by: Shivank Garg <shivankg@amd.com>
>
> <snip>
>
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -1001,11 +1001,17 @@ int filemap_add_folio(struct address_space *mapping, struct folio *folio,
>> EXPORT_SYMBOL_GPL(filemap_add_folio);
>>
>> #ifdef CONFIG_NUMA
>> -struct folio *filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order)
>> +struct folio *filemap_alloc_folio_mpol_noprof(gfp_t gfp, unsigned int order,
>> + struct mempolicy *mpol)
>> {
>> int n;
>> struct folio *folio;
>>
>> + if (mpol)
>> + return folio_alloc_mpol_noprof(gfp, order, mpol,
>> + NO_INTERLEAVE_INDEX,

Could we pass in the interleave index instead of hard-coding it?

>> + numa_node_id());
>> +
>> if (cpuset_do_page_mem_spread()) {
>> unsigned int cpuset_mems_cookie;
>> do {
>> @@ -1018,6 +1024,12 @@ struct folio *filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order)
>> }
>> return folio_alloc_noprof(gfp, order);
>> }
>> +EXPORT_SYMBOL(filemap_alloc_folio_mpol_noprof);
>> +
>> +struct folio *filemap_alloc_folio_noprof(gfp_t gfp, unsigned int order)
>> +{
>> + return filemap_alloc_folio_mpol_noprof(gfp, order, NULL);
>> +}
>> EXPORT_SYMBOL(filemap_alloc_folio_noprof);
>> #endif
>
> Here it seems to me:
>
> - filemap_alloc_folio_noprof() could stay unchanged
> - filemap_alloc_folio_mpol_noprof() would
> - call folio_alloc_mpol_noprof() if (mpol)
> - call filemap_alloc_folio_noprof() otherwise
>
> The code would be a bit more clearly structured that way?
>

I feel that the original proposal makes it clearer that for all filemap
folio allocations, if mpol is defined, anything to do with cpuset's page
spread is overridden. Just a slight preference though. I do also agree
that having filemap_alloc_folio_mpol_noprof() call
filemap_alloc_folio_noprof() would result in fewer changes.

>> @@ -1881,11 +1893,12 @@ void *filemap_get_entry(struct address_space *mapping, pgoff_t index)
>> }
>>
>> /**
>> - * __filemap_get_folio - Find and get a reference to a folio.
>> + * __filemap_get_folio_mpol - Find and get a reference to a folio.
>> * @mapping: The address_space to search.
>> * @index: The page index.
>> * @fgp_flags: %FGP flags modify how the folio is returned.
>> * @gfp: Memory allocation flags to use if %FGP_CREAT is specified.
>> + * @mpol: The mempolicy to apply when allocating a new folio.
>> *
>> * Looks up the page cache entry at @mapping & @index.
>> *
>> @@ -1896,8 +1909,8 @@ void *filemap_get_entry(struct address_space *mapping, pgoff_t index)
>> *
>> * Return: The found folio or an ERR_PTR() otherwise.
>> */
>> -struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
>> - fgf_t fgp_flags, gfp_t gfp)
>> +struct folio *__filemap_get_folio_mpol(struct address_space *mapping, pgoff_t index,
>> + fgf_t fgp_flags, gfp_t gfp, struct mempolicy *mpol)
>> {
>> struct folio *folio;
>>
>> @@ -1967,7 +1980,7 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
>> err = -ENOMEM;
>> if (order > min_order)
>> alloc_gfp |= __GFP_NORETRY | __GFP_NOWARN;
>> - folio = filemap_alloc_folio(alloc_gfp, order);
>> + folio = filemap_alloc_folio_mpol(alloc_gfp, order, mpol);
>> if (!folio)
>> continue;
>>
>> @@ -2003,6 +2016,13 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
>> folio_clear_dropbehind(folio);
>> return folio;
>> }
>> +EXPORT_SYMBOL(__filemap_get_folio_mpol);
>> +
>> +struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
>> + fgf_t fgp_flags, gfp_t gfp)
>> +{
>> + return __filemap_get_folio_mpol(mapping, index, fgp_flags, gfp, NULL);
>> +}
>> EXPORT_SYMBOL(__filemap_get_folio);
>>
>> static inline struct folio *find_get_entry(struct xa_state *xas, pgoff_t max,