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

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On 9/11/24 14:57, David Hildenbrand wrote:
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@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..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.

Makes sense, thanks.

  {
-    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);

Yes, while writing it I knew about this inconsistency, but I wanted
to reduce latency by dropping the lock before. But in do_huge_zero_wp_pmd(),
I couldn't figure out a way to call the stat function after dropping the lock, without I guess, introducing too many labels and goto jumps and the like. In the
current code, the lock gets dropped first.


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.

I will then call it in map_pmd_thp(), that is cleaner...I did find occurrences of these stat computations after taking the lock, for example, in do_swap_page():
count_vm_event(PGMAJFAULT)
so I guess it should be alright.

+
+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().

The code I changed, already was passing vmf->address to update_mmu_cache_pmd(). I did not change any of the haddr and vmf->address semantics, so really can't comment
on this.
I agree with the name change; vmf will be required for set_pmd_at(vmf->pmd), so I should
just pass 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;

Okay.
...

+
+    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.

Are you asking me to align this line with the below line?

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






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