On Mon, Jun 08, 2026 at 04:39:02AM -0400, Michael S. Tsirkin wrote: > Convert alloc_anon_folio() to pass __GFP_ZERO instead of zeroing > at the callsite. post_alloc_hook uses the fault address passed > through vma_alloc_folio for cache-friendly zeroing. > > Note: before this series, replacing clear_user_highpage() with > __GFP_ZERO was unsafe on cache-aliasing architectures because > __GFP_ZERO uses clear_page() without a dcache flush. With this > series, it is safe if the caller passes a valid user address > (not USER_ADDR_NONE) to vma_alloc_folio() etc., which delivers > it to post_alloc_hook() for the dcache flush via > folio_zero_user(). It is only unsafe if USER_ADDR_NONE is passed. > > Note: with __GFP_ZERO, the folio is zeroed before > mem_cgroup_charge(). If the charge fails, the zeroing work is > wasted. Previously zeroing was done after a successful charge. > This is inherent to moving zeroing into the allocator. > Charge failures are rare (only at cgroup limits). > > Use folio_put_zeroed() on charge failure so the zeroed hint > propagates to the buddy allocator, avoiding redundant re-zeroing > on the next allocation attempt.
Is this even worth the effort? This is surely not a hotpath... > > Signed-off-by: Michael S. Tsirkin <[email protected]> > Reviewed-by: Gregory Price <[email protected]> > Assisted-by: Claude:claude-opus-4-6 > --- > mm/memory.c | 13 ++----------- > 1 file changed, 2 insertions(+), 11 deletions(-) > > diff --git a/mm/memory.c b/mm/memory.c > index 6c14b90f558e..6d6a3e1a02c1 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -5265,25 +5265,16 @@ static struct folio *alloc_anon_folio(struct vm_fault > *vmf) > goto fallback; > > /* Try allocating the highest of the remaining orders. */ > - gfp = vma_thp_gfp_mask(vma); > + gfp = vma_thp_gfp_mask(vma) | __GFP_ZERO; > while (orders) { > folio = vma_alloc_folio(gfp, order, vma, vmf->address); > if (folio) { > if (mem_cgroup_charge(folio, vma->vm_mm, gfp)) { > count_mthp_stat(order, > MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE); > - folio_put(folio); > + folio_put_zeroed(folio); You just allocated the folio as zeroed above, should PG_zeroed not be set thus making it unnecessary to add folio_put_zeroed()? > goto next; > } > 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: > -- > MST > Thanks, Lorenzo
