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

On 11.09.24 11:27, David Hildenbrand wrote:

On 11.09.24 08:55, Dev Jain wrote:

quoted

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,

quoted

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.

quoted

   {
-	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.

quoted

+
+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)

... or better "map_anon_folio_pmd" so it better matches 
vma_alloc_folio_pmd() suggested above.

Could also do

vma_map_anon_folio_pmd() :)

-- 
Cheers,

David / dhildenb