Re: [PATCH RFC 3/9] mm: add __GFP_PREZEROED flag and folio_test_clear_prezeroed()

[David Hildenbrand (Arm)]

On 4/13/26 22:37, Michael S. Tsirkin wrote:
> On Mon, Apr 13, 2026 at 11:05:40AM +0200, David Hildenbrand (Arm) wrote:
>> On 4/13/26 00:50, Michael S. Tsirkin wrote:
>>> The previous patch skips zeroing in post_alloc_hook() when
>>> __GFP_ZERO is used.  However, several page allocation paths
>>> zero pages via folio_zero_user() or clear_user_highpage() after
>>> allocation, not via __GFP_ZERO.
>>>
>>> Add __GFP_PREZEROED gfp flag that tells post_alloc_hook() to
>>> preserve the MAGIC_PAGE_ZEROED sentinel in page->private so the
>>> caller can detect pre-zeroed pages and skip its own zeroing.
>>> Add folio_test_clear_prezeroed() helper to check and clear
>>> the sentinel.
>>
>> I really don't like __GFP_PREZEROED, and wonder how we can avoid it.
>>
>>
>> What you want is, allocate a folio (well, actually a page that becomes
>> a folio) and know whether zeroing for that folio (once we establish it
>> from a page) is still required.
>>
>> Or you just allocate a folio, specify GFP_ZERO, and let the folio
>> allocation code deal with that.
>>
>>
>> I think we have two options:
>>
>> (1) Use an indication that can be sticky for callers that do not care.
>>
>> Assuming we would use a page flag that is only ever used on folios, all
>> we'd have to do is make sure that we clear the flag once we convert
>> the to a folio.
>>
>> For example, PG_dropbehind is only ever set on folios in the pagecache. 
>>
>> Paths that allocate folios would have to clear the flag. For non-hugetlb
>> folios that happens through page_rmappable_folio().
>>
>> I'm not super-happy about that, but it would be doable.
>>
>>
>> (2) Use a dedicated allocation interface for user pages in the buddy.
>>
>> I hate the whole user_alloc_needs_zeroing()+folio_zero_user() handling.
>>
>> It shouldn't exist. We should just be passing GFP_ZERO and let the buddy handle
>> all that.
>>
>>
>> For example, vma_alloc_folio() already gets passed the address in.
>>
>> Pass the address from vma_alloc_folio_noprof()->folio_alloc_noprof(), and let
>> folio_alloc_noprof() use a buddy interface that can handle it.
>>
>> Imagine if we had a alloc_user_pages_noprof() that consumes an address. It could just
>> do what folio_zero_user() does, and only if really required.
>>
>> The whole user_alloc_needs_zeroing() could go away and you could just handle the
>> pre-zeroed optimization internally.
>>
>> -- 
>> Cheers,
>>
>> David
> 
> I admit I only vaguely understand the core mm refactoring you are suggesting.
> 

Oh, I was hoping claude would figure that out for you.


Essentially, we move the zeroing of folios back into the buddy, by using
GFP_ZERO.

user_alloc_needs_zeroing() logic would reside in the buddy and is no longer
required in callers.

E.g.,

diff --git a/mm/memory.c b/mm/memory.c
index 631205a384e1..44576ba3def5 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5259,7 +5259,7 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
        gfp = vma_thp_gfp_mask(vma);
        while (orders) {
                addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
-               folio = vma_alloc_folio(gfp, order, vma, addr);
+               folio = vma_alloc_folio(gfp | GFP_ZERO, order, vma, addr);
                if (!folio)
                        goto next;
                if (mem_cgroup_charge(folio, vma->vm_mm, gfp)) {
@@ -5272,15 +5272,6 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
                        goto fallback;
                }
                folio_throttle_swaprate(folio, gfp);
-               /*
-                * When a folio is not zeroed during allocation
-                * (__GFP_ZERO not used) or user folios require special
-                * handling, folio_zero_user() is used to make sure
-                * that the page corresponding to the faulting address
-                * will be hot in the cache after zeroing.
-                */
-               if (user_alloc_needs_zeroing())
-                       folio_zero_user(folio, vmf->address);
                return folio;
 next:
                count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);


folio_zero_user(), from where we would extract a function that operates on a
page+order chunk, requires the address hint.

So we would have to pass that address. For example for the !CONFIG_NUMA case,
something like the following could be done.

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index 51ef13ed756e..29771c3240be 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -234,6 +234,10 @@ struct folio *__folio_alloc_noprof(gfp_t gfp, unsigned int order, int preferred_
                nodemask_t *nodemask);
 #define __folio_alloc(...)                     alloc_hooks(__folio_alloc_noprof(__VA_ARGS__))
 
+struct folio *__folio_alloc_user_noprof(gfp_t gfp, unsigned int order, int preferred_nid,
+               nodemask_t *nodemask, unsigned long addr);
+#define __folio_alloc_user(...)                        alloc_hooks(__folio_alloc_user_noprof(__VA_ARGS__))
+
 unsigned long alloc_pages_bulk_noprof(gfp_t gfp, int preferred_nid,
                                nodemask_t *nodemask, int nr_pages,
                                struct page **page_array);
@@ -291,6 +295,18 @@ __alloc_pages_node_noprof(int nid, gfp_t gfp_mask, unsigned int order)
 
 #define  __alloc_pages_node(...)               alloc_hooks(__alloc_pages_node_noprof(__VA_ARGS__))
 
+static inline
+struct folio *__folio_alloc_user_node_noprof(gfp_t gfp, unsigned int order,
+               int nid, unsigned long addr)
+{
+       VM_BUG_ON(nid < 0 || nid >= MAX_NUMNODES);
+       warn_if_node_offline(nid, gfp);
+
+       return __folio_alloc_user_noprof(gfp, order, nid, NULL, addr);
+}
+
+#define  __folio_alloc_user_node(...)          alloc_hooks(__folio_alloc_user_node_noprof(__VA_ARGS__))
+
 static inline
 struct folio *__folio_alloc_node_noprof(gfp_t gfp, unsigned int order, int nid)
 {
@@ -342,7 +358,7 @@ static inline struct folio *folio_alloc_mpol_noprof(gfp_t gfp, unsigned int orde
 static inline struct folio *vma_alloc_folio_noprof(gfp_t gfp, int order,
                struct vm_area_struct *vma, unsigned long addr)
 {
-       return folio_alloc_noprof(gfp, order);
+       return __folio_alloc_user_node_noprof(gfp, order, numa_node_id(), addr);
 }
 #endif
 
index ee81f5c67c18..28f448f40b75 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5260,6 +5260,13 @@ struct folio *__folio_alloc_noprof(gfp_t gfp, unsigned int order, int preferred_
 }
 EXPORT_SYMBOL(__folio_alloc_noprof);
 
+struct folio *__folio_alloc_user_noprof(gfp_t gfp, unsigned int order, int preferred_nid,
+               nodemask_t *nodemask, unsigned long addr)
+{
+       /* TODO */
+}
+EXPORT_SYMBOL(__folio_alloc_noprof);
+
 /*
  * Common helper functions. Never use with __GFP_HIGHMEM because the returned
  * address cannot represent highmem pages. Use alloc_pages and then kmap if


As alloc_user_pages() resides in the buddy, it can just honor any
buddy-internal "pre-zeroed" flag.


Once you are in page_alloc.c you can access internal allocation functions and
take care of that without GFP flags.

-- 
Cheers,

David