On 06/09/2024 09:45, Dev Jain wrote:
>
> On 9/6/24 13:58, Ryan Roberts wrote:
>> On 06/09/2024 06:42, Dev Jain wrote:
>>> On 9/5/24 16:38, Ryan Roberts wrote:
>>>> On 04/09/2024 11:09, Dev Jain wrote:
>>>>> In preparation for the second patch, abstract away the THP allocation
>>>>> logic present in the create_huge_pmd() path, which corresponds to the
>>>>> faulting case when no page is present.
>>>>>
>>>>> There should be no functional change as a result of applying
>>>>> this patch.
>>>>>
>>>>> Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
>>>>> ---
>>>>> mm/huge_memory.c | 110 +++++++++++++++++++++++++++++------------------
>>>>> 1 file changed, 67 insertions(+), 43 deletions(-)
>>>>>
>>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>>> index 67c86a5d64a6..58125fbcc532 100644
>>>>> --- a/mm/huge_memory.c
>>>>> +++ b/mm/huge_memory.c
>>>>> @@ -943,47 +943,89 @@ unsigned long thp_get_unmapped_area(struct file *filp,
>>>>> unsigned long addr,
>>>>> }
>>>>> EXPORT_SYMBOL_GPL(thp_get_unmapped_area);
>>>>> -static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
>>>>> - struct page *page, gfp_t gfp)
>>>>> +static vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct
>>>>> vm_area_struct *vma,
>>>> Is there a reason for specifying order as a parameter? Previously it was
>>>> hardcoded to HPAGE_PMD_ORDER. But now, thp_fault_alloc() and
>>>> __thp_fault_success_stats() both take order and map_pmd_thp() is still
>>>> implicitly mapping a PMD-sized block. Unless there is a reason you need this
>>>> parameter in the next patch (I don't think there is?) I suggest simplifying.
>>> If I am not wrong, thp_fault_alloc() and __thp_fault_success_stats()
>>> will remain the same in case of mTHP?
>> No, its a bit different for smaller-than-pmd THP - that's handled in
>> alloc_anon_folio() in memory.c. It deals with fallback to smaller orders (inc
>> order-0. Potentially alloc_anon_folio() could be refactored to use your new
>> functions in future but I'm sure there would be a number of subtlties to
>> consider and in any case you would want to make that a separate change. The
>> order param would be added as part of that future change.
>
> Okay.
>
>>
>>> I chose to pass order so that these
>>> functions can be used by others in the future.
>> It's not valuable to have an internal interface with no users. My advice is to
>> keep it simple and only extend it when a user pops up.
>
> Makes sense. In that case, should I call it pmd_thp_fault_alloc() to enforce that
> this is not a non-PMD THP?
Yeah, that works for me.
>
>>
>>> Therefore, these two functions
>>> can be generically used in the future while map_pmd_thp() (as the name suggests)
>>> maps only a PMD-mappable THP.
>>>
>>>>> + unsigned long haddr, struct folio **foliop,
>>>> FWIW, I agree with Kirill's suggestion to just return folio* and drop the
>>>> output
>>>> param.
>>> As I replied to Kirill, I do not think that is a good idea. If you do a git
>>> grep on
>>> the tree for "foliop", you will find several places where that is being used,
>>> for
>>> example, check out alloc_charge_folio() in mm/khugepaged.c. The author
>>> intends to
>>> do the stat computation and setting *foliop in the function itself, so that the
>>> caller is only concerned with the return value.
>> By having 2 return params, you open up the possibility of returning an invalid
>> combination (e.g. ret==FALLBACK, folio!=NULL). You avoid that by having a single
>> return param, which is either NULL (failure) or a valid pointer.
>
> Okay, I agree with this in a generic sense.
>
>>
>>> Also, if we return the folio instead of VM_FAULT_FALLBACK from
>>> thp_fault_alloc(),
>>> then you will have two "if (unlikely(!folio))" branches, the first to do the
>>> stat
>>> computation in the function, and the second branch in the caller to set ret =
>>> VM_FAULT_FALLBACK
>>> and then goto out/release.
>> There are already 2 conditionals, that doesn't change.
>>
>>> And, it is already inconsistent to break away the stat computation and the
>>> return value
>>> setting, when the stat computation name is of the form
>>> "count_vm_event(THP_FAULT_FALLBACK)"
>>> and friends, at which point it would just be better to open code the function.
>> I don't understand this.
>>
>>> If I am missing something and your suggestion can be implemented neatly, please
>>> guide.
>> This looks cleaner/simpler to my eye (also removing the order parameter):
>>
>> static folio *thp_fault_alloc(gfp_t gfp, struct vm_area_struct *vma,
>> unsigned long haddr, unsigned long addr)
>> {
>> const int order = HPAGE_PMD_ORDER;
>> struct folio *folio = vma_alloc_folio(gfp, order, vma, haddr, true);
>>
>> if (unlikely(!folio)) {
>> count_vm_event(THP_FAULT_FALLBACK);
>> count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
>> goto out;
>> }
>>
>> VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
>> if (mem_cgroup_charge(folio, vma->vm_mm, gfp)) {
>> folio_put(folio);
>> count_vm_event(THP_FAULT_FALLBACK);
>> count_vm_event(THP_FAULT_FALLBACK_CHARGE);
>> count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
>> count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
>> goto out;
>> }
>> folio_throttle_swaprate(folio, gfp);
>>
>> 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()
>> * write.
>> */
>> __folio_mark_uptodate(folio);
>> out:
>> return folio;
>> }
>>
>>
>> static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>> {
>> vm_fault_t ret = VM_FAULT_FALLBACK;
>> ...
>>
>> folio = thp_fault_alloc(gfp, vma, haddr, vmf->address);
>> if (unlikely(!folio))
>> goto release;
>> ...
>> }
>>
>
> Thanks for your help. I will switch to this, just that, ret should still
> be initialized to zero in case we spot the pmd changes after taking the lock.
> So I'll set it in the case of !folio.
ret = thp_fault_alloc() will have already set it back to zero so there isn't a
bug. But if you prefer to explicitly set it in the "if" to make the code clearer
to read, that's fine by me.
Thanks,
Ryan
>
>>
>>>> Thanks,
>>>> Ryan
>>>>
>>>>> + unsigned long addr)
>>>>> {
>>>>> - struct vm_area_struct *vma = vmf->vma;
>>>>> - struct folio *folio = page_folio(page);
>>>>> - pgtable_t pgtable;
>>>>> - unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
>>>>> - vm_fault_t ret = 0;
>>>>> + struct folio *folio = vma_alloc_folio(gfp, order, vma, haddr, true);
>>>>> - VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
>>>>> + *foliop = folio;
>>>>> + if (unlikely(!folio)) {
>>>>> + count_vm_event(THP_FAULT_FALLBACK);
>>>>> + count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
>>>>> + return VM_FAULT_FALLBACK;
>>>>> + }
>>>>> + VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
>>>>> if (mem_cgroup_charge(folio, vma->vm_mm, gfp)) {
>>>>> folio_put(folio);
>>>>> count_vm_event(THP_FAULT_FALLBACK);
>>>>> count_vm_event(THP_FAULT_FALLBACK_CHARGE);
>>>>> - count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_FALLBACK);
>>>>> - count_mthp_stat(HPAGE_PMD_ORDER,
>>>>> MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
>>>>> + count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
>>>>> + count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
>>>>> return VM_FAULT_FALLBACK;
>>>>> }
>>>>> folio_throttle_swaprate(folio, gfp);
>>>>> - pgtable = pte_alloc_one(vma->vm_mm);
>>>>> - if (unlikely(!pgtable)) {
>>>>> - ret = VM_FAULT_OOM;
>>>>> - goto release;
>>>>> - }
>>>>> -
>>>>> - folio_zero_user(folio, vmf->address);
>>>>> + 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()
>>>>> * write.
>>>>> */
>>>>> __folio_mark_uptodate(folio);
>>>>> + return 0;
>>>>> +}
>>>>> +
>>>>> +static void __thp_fault_success_stats(struct vm_area_struct *vma, int order)
>>>>> +{
>>>>> + count_vm_event(THP_FAULT_ALLOC);
>>>>> + count_mthp_stat(order, MTHP_STAT_ANON_FAULT_ALLOC);
>>>>> + count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
>>>>> +}
>>>>> +
>>>>> +static void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
>>>>> + struct vm_area_struct *vma, unsigned long haddr,
>>>>> + pgtable_t pgtable)
>>>>> +{
>>>>> + pmd_t entry;
>>>>> +
>>>>> + entry = mk_huge_pmd(&folio->page, vma->vm_page_prot);
>>>>> + entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
>>>>> + folio_add_new_anon_rmap(folio, vma, haddr, RMAP_EXCLUSIVE);
>>>>> + folio_add_lru_vma(folio, vma);
>>>>> + pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
>>>>> + set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
>>>>> + update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
>>>>> + add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
>>>>> + mm_inc_nr_ptes(vma->vm_mm);
>>>>> +}
>>>>> +
>>>>> +static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>>>>> +{
>>>>> + struct vm_area_struct *vma = vmf->vma;
>>>>> + struct folio *folio = NULL;
>>>>> + pgtable_t pgtable;
>>>>> + unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
>>>>> + vm_fault_t ret = 0;
>>>>> + gfp_t gfp = vma_thp_gfp_mask(vma);
>>>>> +
>>>>> + pgtable = pte_alloc_one(vma->vm_mm);
>>>>> + if (unlikely(!pgtable)) {
>>>>> + ret = VM_FAULT_OOM;
>>>>> + goto release;
>>>>> + }
>>>>> +
>>>>> + ret = thp_fault_alloc(gfp, HPAGE_PMD_ORDER, vma, haddr, &folio,
>>>>> + vmf->address);
>>>>> + if (ret)
>>>>> + goto release;
>>>>> vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
>>>>> +
>>>>> if (unlikely(!pmd_none(*vmf->pmd))) {
>>>>> goto unlock_release;
>>>>> } else {
>>>>> - pmd_t entry;
>>>>> -
>>>>> ret = check_stable_address_space(vma->vm_mm);
>>>>> if (ret)
>>>>> goto unlock_release;
>>>>> @@ -997,20 +1039,9 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct
>>>>> vm_fault *vmf,
>>>>> VM_BUG_ON(ret & VM_FAULT_FALLBACK);
>>>>> return ret;
>>>>> }
>>>>> -
>>>>> - entry = mk_huge_pmd(page, vma->vm_page_prot);
>>>>> - entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
>>>>> - folio_add_new_anon_rmap(folio, vma, haddr, RMAP_EXCLUSIVE);
>>>>> - folio_add_lru_vma(folio, vma);
>>>>> - pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
>>>>> - set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
>>>>> - update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
>>>>> - add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
>>>>> - mm_inc_nr_ptes(vma->vm_mm);
>>>>> + map_pmd_thp(folio, vmf, vma, haddr, pgtable);
>>>>> spin_unlock(vmf->ptl);
>>>>> - 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);
>>>>> + __thp_fault_success_stats(vma, HPAGE_PMD_ORDER);
>>>>> }
>>>>> return 0;
>>>>> @@ -1019,7 +1050,8 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct
>>>>> vm_fault *vmf,
>>>>> release:
>>>>> if (pgtable)
>>>>> pte_free(vma->vm_mm, pgtable);
>>>>> - folio_put(folio);
>>>>> + if (folio)
>>>>> + folio_put(folio);
>>>>> return ret;
>>>>> }
>>>>> @@ -1077,8 +1109,6 @@ static void set_huge_zero_folio(pgtable_t pgtable,
>>>>> struct mm_struct *mm,
>>>>> vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>>>>> {
>>>>> struct vm_area_struct *vma = vmf->vma;
>>>>> - gfp_t gfp;
>>>>> - struct folio *folio;
>>>>> unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
>>>>> vm_fault_t ret;
>>>>> @@ -1129,14 +1159,8 @@ vm_fault_t do_huge_pmd_anonymous_page(struct
>>>>> vm_fault *vmf)
>>>>> }
>>>>> return ret;
>>>>> }
>>>>> - gfp = vma_thp_gfp_mask(vma);
>>>>> - folio = vma_alloc_folio(gfp, HPAGE_PMD_ORDER, vma, haddr, true);
>>>>> - if (unlikely(!folio)) {
>>>>> - count_vm_event(THP_FAULT_FALLBACK);
>>>>> - count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_FALLBACK);
>>>>> - return VM_FAULT_FALLBACK;
>>>>> - }
>>>>> - return __do_huge_pmd_anonymous_page(vmf, &folio->page, gfp);
>>>>> +
>>>>> + return __do_huge_pmd_anonymous_page(vmf);
>>>>> }
>>>>> static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long
>>>>> addr,
