On Mon, Jun 08, 2026 at 04:39:18AM -0400, Michael S. Tsirkin wrote: > Convert vma_alloc_anon_folio_pmd() 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 folio_zero_user() 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.
Again, is this worth it?... Every bit of code added increases risks of bugs, maintenance burden, etc. let's just not do stuff because we can. > > Signed-off-by: Michael S. Tsirkin <[email protected]> > Reviewed-by: Gregory Price <[email protected]> > Assisted-by: Claude:claude-opus-4-6 > --- > mm/huge_memory.c | 14 +++----------- > 1 file changed, 3 insertions(+), 11 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index d689e6491ddb..0dec3c717ff2 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -1333,7 +1333,7 @@ EXPORT_SYMBOL_GPL(thp_get_unmapped_area); > static struct folio *vma_alloc_anon_folio_pmd(struct vm_area_struct *vma, > unsigned long addr) > { > - gfp_t gfp = vma_thp_gfp_mask(vma); > + gfp_t gfp = vma_thp_gfp_mask(vma) | __GFP_ZERO; > const int order = HPAGE_PMD_ORDER; > struct folio *folio; > > @@ -1347,7 +1347,7 @@ static struct folio *vma_alloc_anon_folio_pmd(struct > vm_area_struct *vma, > > VM_BUG_ON_FOLIO(!folio_test_large(folio), folio); > if (mem_cgroup_charge(folio, vma->vm_mm, gfp)) { > - folio_put(folio); > + folio_put_zeroed(folio); Same comments as previously. > count_vm_event(THP_FAULT_FALLBACK); > count_vm_event(THP_FAULT_FALLBACK_CHARGE); > count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK); > @@ -1356,17 +1356,9 @@ static struct folio *vma_alloc_anon_folio_pmd(struct > vm_area_struct *vma, > } > 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, addr); > /* > * The memory barrier inside __folio_mark_uptodate makes sure that > - * folio_zero_user writes become visible before the set_pmd_at() > + * page zeroing becomes visible before the set_pmd_at() folio zeroing? > * write. > */ > __folio_mark_uptodate(folio); > -- > MST > Thanks, Lorenzo
