mm/hugetlb: simplify vma_has_reserves()

vma_has_reserves() is a helper "trying" to know whether the vma should
consume one reservation when allocating the hugetlb folio.

However it's not clear on why we need such complexity, as such information
is already represented in the "chg" variable.

From alloc_hugetlb_folio() context, "chg" (or in the function's context,
"gbl_chg") is defined as:

  - If gbl_chg=1, the allocation cannot reuse an existing reservation
  - If gbl_chg=0, the allocation should reuse an existing reservation

Firstly, map_chg is defined as following, to cover all cases of hugetlb
reservation scenarios (mostly, via vma_needs_reservation(), but
cow_from_owner is an outlier):

CONDITION                                             HAS RESERVATION?
=========                                             ================
- SHARED: always check against per-inode resv_map
  (ignore NONRESERVE)
  - If resv exists                                ==> YES  [1]
  - If not                                        ==> NO   [2]
- PRIVATE: complicated...
  - Request came from a CoW from owner resv map   ==> NO   [3]
    (when cow_from_owner==true)
  - If does not own a resv_map at all..           ==> NO   [4]
    (examples: VM_NORESERVE, private fork())
  - If owns a resv_map, but resv donsn't exists   ==> NO   [5]
  - If owns a resv_map, and resv exists           ==> YES  [6]

Further on, gbl_chg considered spool setup, so that is a decision based on
all the context.

If we look at vma_has_reserves(), it almost does check that has already
been processed by map_chg accounting (I marked each return value to the
case above):

  static bool vma_has_reserves(struct vm_area_struct *vma, long chg)
  {
          if (vma->vm_flags & VM_NORESERVE) {
                  if (vma->vm_flags & VM_MAYSHARE && chg == 0)
                          return true;              ==> [1]
                  else
                          return false;             ==> [2] or [4]
          }

          if (vma->vm_flags & VM_MAYSHARE) {
                  if (chg)
                          return false;             ==> [2]
                  else
                          return true;              ==> [1]
          }

          if (is_vma_resv_set(vma, HPAGE_RESV_OWNER)) {
                  if (chg)
                          return false;             ==> [5]
                  else
                          return true;              ==> [6]
          }

          return false;                             ==> [4]
  }

It didn't check [3], but [3] case was actually already covered now by the
"chg" / "gbl_chg" / "map_chg" calculations.

In short, vma_has_reserves() doesn't provide anything more than return
"!chg"..  so just simplify all the things.

There're a lot of comments describing truncation races, IIUC there should
have no race as long as map_chg is properly done.

Link: https://lkml.kernel.org/r/20250107204002.2683356-6-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: Ackerley Tng <ackerleytng@google.com>
Cc: Breno Leitao <leitao@debian.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Rik van Riel <riel@surriel.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index bac6ee9ede4d2a78311f183f0f02f32562436f25..efd8e9f9bf0ea71d51f75d2f68bce5a972725ecd 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1248,66 +1248,13 @@ void clear_vma_resv_huge_pages(struct vm_area_struct *vma)
 }
 
 /* Returns true if the VMA has associated reserve pages */
-static bool vma_has_reserves(struct vm_area_struct *vma, long chg)
+static bool vma_has_reserves(long chg)
 {
-       if (vma->vm_flags & VM_NORESERVE) {
-               /*
-                * This address is already reserved by other process(chg == 0),
-                * so, we should decrement reserved count. Without decrementing,
-                * reserve count remains after releasing inode, because this
-                * allocated page will go into page cache and is regarded as
-                * coming from reserved pool in releasing step.  Currently, we
-                * don't have any other solution to deal with this situation
-                * properly, so add work-around here.
-                */
-               if (vma->vm_flags & VM_MAYSHARE && chg == 0)
-                       return true;
-               else
-                       return false;
-       }
-
-       /* Shared mappings always use reserves */
-       if (vma->vm_flags & VM_MAYSHARE) {
-               /*
-                * We know VM_NORESERVE is not set.  Therefore, there SHOULD
-                * be a region map for all pages.  The only situation where
-                * there is no region map is if a hole was punched via
-                * fallocate.  In this case, there really are no reserves to
-                * use.  This situation is indicated if chg != 0.
-                */
-               if (chg)
-                       return false;
-               else
-                       return true;
-       }
-
        /*
-        * Only the process that called mmap() has reserves for
-        * private mappings.
+        * Now "chg" has all the conditions considered for whether we
+        * should use an existing reservation.
         */
-       if (is_vma_resv_set(vma, HPAGE_RESV_OWNER)) {
-               /*
-                * Like the shared case above, a hole punch or truncate
-                * could have been performed on the private mapping.
-                * Examine the value of chg to determine if reserves
-                * actually exist or were previously consumed.
-                * Very Subtle - The value of chg comes from a previous
-                * call to vma_needs_reserves().  The reserve map for
-                * private mappings has different (opposite) semantics
-                * than that of shared mappings.  vma_needs_reserves()
-                * has already taken this difference in semantics into
-                * account.  Therefore, the meaning of chg is the same
-                * as in the shared case above.  Code could easily be
-                * combined, but keeping it separate draws attention to
-                * subtle differences.
-                */
-               if (chg)
-                       return false;
-               else
-                       return true;
-       }
-
-       return false;
+       return chg == 0;
 }
 
 static void enqueue_hugetlb_folio(struct hstate *h, struct folio *folio)
@@ -1411,7 +1358,7 @@ static struct folio *dequeue_hugetlb_folio_vma(struct hstate *h,
         * have no page reserves. This check ensures that reservations are
         * not "stolen". The child may still get SIGKILLed
         */
-       if (!vma_has_reserves(vma, chg) && !available_huge_pages(h))
+       if (!vma_has_reserves(chg) && !available_huge_pages(h))
                goto err;
 
        gfp_mask = htlb_alloc_mask(h);
@@ -1429,7 +1376,7 @@ static struct folio *dequeue_hugetlb_folio_vma(struct hstate *h,
                folio = dequeue_hugetlb_folio_nodemask(h, gfp_mask,
                                                        nid, nodemask);
 
-       if (folio && vma_has_reserves(vma, chg)) {
+       if (folio && vma_has_reserves(chg)) {
                folio_set_hugetlb_restore_reserve(folio);
                h->resv_huge_pages--;
        }
@@ -3120,7 +3067,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
                if (!folio)
                        goto out_uncharge_cgroup;
                spin_lock_irq(&hugetlb_lock);
-               if (vma_has_reserves(vma, gbl_chg)) {
+               if (vma_has_reserves(gbl_chg)) {
                        folio_set_hugetlb_restore_reserve(folio);
                        h->resv_huge_pages--;
                }