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This should be the same for a pmd folio during wp >>>> page fault. >>>> >>>> Commit 1ced09e0331f ("mm: allocate THP on hugezeropage wp-fault") mi= ss >>>> to add it to ds_queue, which means system may not reclaim enough mem= ory >>> >>> IIRC, it was commit dafff3f4c850 ("mm: split underused THPs") that >>> started unconditionally adding all new anon THPs to _deferred_list :)= >>> >>>> in case of memory pressure even the pmd folio is under used. >>>> >>>> Move deferred_split_folio() into map_anon_folio_pmd() to make the pm= d >>>> folio installation consistent. >>>> >>>> Fixes: 1ced09e0331f ("mm: allocate THP on hugezeropage wp-fault") >>> >>> Shouldn't this rather be the following? >>> >>> Fixes: dafff3f4c850 ("mm: split underused THPs") >> >> Yes, I agree. In this case, this patch looks more like an optimization= >> for split underused THPs. >> >> One observation on this change is that right after zero pmd wp, the >> deferred split queue could be scanned, the newly added pmd folio will >> split since it is all zero except one subpage. This means we probably >> should allocate a base folio for zero pmd wp and map the rest to zero >> page at the beginning if split underused THP is enabled to avoid >> this long trip. The downside is that user app cannot get a pmd folio >> if it is intended to write data into the entire folio. >> >> Usama might be able to give some insight here. >> > > Thanks for CCing me Zi! > > hmm I think the downside of not having PMD folio probably outweights th= e cost of splitting > a zer-filled page? Yeah, I agree. > ofcourse I dont have any numbers to back that up, but that would be my = initial guess. > > Also: > > Acked-by: Usama Arif > > >> >>> >>> Thanks, >>> Lance >>> >>>> Signed-off-by: Wei Yang >>>> Cc: David Hildenbrand >>>> Cc: Lance Yang >>>> Cc: Dev Jain >>>> Cc: >>>> >>>> --- >>>> v2: >>>> * add fix, cc stable and put description about the flow of curren= t >>>> code >>>> * move deferred_split_folio() into map_anon_folio_pmd() >>>> --- >>>> mm/huge_memory.c | 2 +- >>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>> >>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>>> index 1b81680b4225..f13de93637bf 100644 >>>> --- a/mm/huge_memory.c >>>> +++ b/mm/huge_memory.c >>>> @@ -1232,6 +1232,7 @@ static void map_anon_folio_pmd(struct folio *f= olio, pmd_t *pmd, >>>> count_vm_event(THP_FAULT_ALLOC); >>>> count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_ALLOC); >>>> count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC); >>>> + deferred_split_folio(folio, false); >>>> } >>>> static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *v= mf) >>>> @@ -1272,7 +1273,6 @@ static vm_fault_t __do_huge_pmd_anonymous_page= (struct vm_fault *vmf) >>>> pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable); >>>> map_anon_folio_pmd(folio, vmf->pmd, vma, haddr); >>>> mm_inc_nr_ptes(vma->vm_mm); >>>> - deferred_split_folio(folio, false); >>>> spin_unlock(vmf->ptl); >>>> } >>>> >> >> >> Best Regards, >> Yan, Zi Best Regards, Yan, Zi