Re: [PATCH v3 1/2] mm: Abstract THP allocation

[David Hildenbrand <david@redhat.com>]

On 11.09.24 08:55, 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.
> 

Hi,

> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>   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..b96a1ff2bf40 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -943,47 +943,88 @@ 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 struct folio *pmd_thp_fault_alloc(gfp_t gfp, struct vm_area_struct *vma,
> +					 unsigned long haddr, unsigned long addr)

I suggest calling this something like "vma_alloc_anon_folio_pmd()"? Then 
it's more consistent with vma_alloc_folio().

Also, likely we should just only pass in "addr" and calculate "haddr" 
ourselves, it's cheap and reduces the number of function parameters.

>   {
> -	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;
> +	const int order = HPAGE_PMD_ORDER;
> +	struct folio *folio = vma_alloc_folio(gfp, order, vma, haddr, true);
>   
> -	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
> +	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(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_FALLBACK);
> -		count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
> -		return VM_FAULT_FALLBACK;
> +		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);
>   
> -	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);
> +out:
> +	return folio;
> +}
> +
> +static void __pmd_thp_fault_success_stats(struct vm_area_struct *vma)
> +{
> +	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);
> +}

Why isn't that moved into map_pmd_thp()

Note that in this patch you do:

map_pmd_thp(folio, vmf, vma, haddr);
spin_unlock(vmf->ptl);
__pmd_thp_fault_success_stats(vma);

But in patch #2

map_pmd_thp(folio, vmf, vma, haddr);
__pmd_thp_fault_success_stats(vma);
goto unlock;
release:
	folio_put(folio);
unlock:
	spin_unlock(vmf->ptl);

Please make that consistent, meaning:

1) Inline __pmd_thp_fault_success_stats() into map_pmd_thp(). No need to 
have the separated out.

2) Either do the PTL unlocking in __pmd_thp_fault_success_stats() or in
    the caller. In the caller is likely easiest. Adjusting the counters
    should be cheap, if not we could revisit this later with real data.

> +
> +static void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
> +			struct vm_area_struct *vma, unsigned long haddr)
> +{
> +	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);
> +	set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
> +	update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);

It's quite weird to see a mixture of haddr and vmf->address, and likely 
this mixture is wrong or not not required.

Looking at arc's update_mmu_cache_pmd() implementation, I cannot see how 
passing in the unaligned address would do the right thing. But maybe arc 
also doesn't trigger that code path ... who knows :)


Staring at some other update_mmu_cache_pmd() users, it's quite 
inconsistent. Primarily only do_huge_pmd_numa_page() and 
__do_huge_pmd_anonymous_page() use the unaligned address. The others 
seem to use the aligned address ... as one would expect when modifying a 
PMD.


I suggest to change this function to *not* pass in the vmf, and rename 
it to something like:

static void folio_map_anon_pmd(struct folio *folio, struct 
vm_area_struct *vma, pmd_t *pmd, unsigned long haddr)

Then use haddr also to do the update_mmu_cache_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;
> +	pgtable_t pgtable;
> +	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
> +	vm_fault_t ret = 0;
> +	gfp_t gfp = vma_thp_gfp_mask(vma);

Nit: While at it, try to use reverse christmas-tree where possible, 
makes things more reasible. You could make haddr const.

struct vm_area_struct *vma = vmf->vma;
unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
gfp_t gfp = vma_thp_gfp_mask(vma);
struct folio *folio;
vm_fault_t ret = 0;
...

> +
> +	folio = pmd_thp_fault_alloc(gfp, vma, haddr, vmf->address);
> +	if (unlikely(!folio)) {
> +		ret = VM_FAULT_FALLBACK;
> +		goto release;
> +	}
> +
> +	pgtable = pte_alloc_one(vma->vm_mm);
> +	if (unlikely(!pgtable)) {
> +		ret = VM_FAULT_OOM;
> +		goto release;
> +	}
>   
>   	vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
> +

Nit Unrelated change.

>   	if (unlikely(!pmd_none(*vmf->pmd))) {
>   		goto unlock_release;


-- 
Cheers,

David / dhildenb