Re: [PATCH v2 19/37] mm/gup: remove record_subpages()

Sat, 06 Sep 2025 22:14:49 -0700 

On 9/5/25 11:56 PM, David Hildenbrand wrote:

On 06.09.25 03:05, John Hubbard wrote:

On 9/1/25 8:03 AM, David Hildenbrand wrote:
...> Well, there is a lot I dislike about record_subpages() to go back there. 
Starting with "as Willy keeps explaining, the concept of subpages do
not exist and ending with "why do we fill out the array even on failure".

:)


I am also very glad to see the entire concept of subpages disappear.



Now it's been returned to it's original, cryptic form.



The code in the caller was so uncryptic that both me and Lorenzo missed
that magical addition. :P


Just my take on it, for whatever that's worth. :)


As always, appreciated.

I could of course keep the simple loop in some "record_folio_pages"
function and clean up what I dislike about record_subpages().
But I much rather want the call chain to be cleaned up instead, if possible. 



Right! The primary way that record_subpages() helped was in showing
what was going on: a function call helps a lot to self-document,
sometimes.
Roughly, what I am thinking (limiting it to pte+pmd case) about is the following: 

The code below looks much cleaner, that's great!

thanks,
--
John Hubbard




 From d6d6d21dbf435d8030782a627175e36e6c7b2dfb Mon Sep 17 00:00:00 2001
From: David Hildenbrand <da...@redhat.com>
Date: Sat, 6 Sep 2025 08:33:42 +0200
Subject: [PATCH] tmp

Signed-off-by: David Hildenbrand <da...@redhat.com>
---
  mm/gup.c | 79 ++++++++++++++++++++++++++------------------------------
  1 file changed, 36 insertions(+), 43 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 22420f2069ee1..98907ead749c0 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2845,12 +2845,11 @@ static void __maybe_unused gup_fast_undo_dev_pagemap(int *nr, int nr_start, 
   * also check pmd here to make sure pmd doesn't change (corresponds to
   * pmdp_collapse_flush() in the THP collapse code path).
   */
-static int gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
-        unsigned long end, unsigned int flags, struct page **pages,
-        int *nr)
+static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr, 
+        unsigned long end, unsigned int flags, struct page **pages)
  {
      struct dev_pagemap *pgmap = NULL;
-    int ret = 0;
+    unsigned long nr_pages = 0;
      pte_t *ptep, *ptem;

      ptem = ptep = pte_offset_map(&pmd, addr);
@@ -2908,24 +2907,20 @@ static int gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr, 
           * details.
           */
          if (flags & FOLL_PIN) {
-            ret = arch_make_folio_accessible(folio);
-            if (ret) {
+            if (arch_make_folio_accessible(folio)) {
                  gup_put_folio(folio, 1, flags);
                  goto pte_unmap;
              }
          }
          folio_set_referenced(folio);
-        pages[*nr] = page;
-        (*nr)++;
+        pages[nr_pages++] = page;
      } while (ptep++, addr += PAGE_SIZE, addr != end);

-    ret = 1;
-
  pte_unmap:
      if (pgmap)
          put_dev_pagemap(pgmap);
      pte_unmap(ptem);
-    return ret;
+    return nr_pages;
  }
  #else
@@ -2938,21 +2933,24 @@ static int gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,   * get_user_pages_fast_only implementation that can pin pages. Thus it's still 
   * useful to have gup_fast_pmd_leaf even if we can't operate on ptes.
   */
-static int gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
-        unsigned long end, unsigned int flags, struct page **pages,
-        int *nr)
+static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr, 
+        unsigned long end, unsigned int flags, struct page **pages)
  {
      return 0;
  }
  #endif /* CONFIG_ARCH_HAS_PTE_SPECIAL */

-static int gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr,
-        unsigned long end, unsigned int flags, struct page **pages,
-        int *nr)
+static unsigned long gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr, 
+        unsigned long end, unsigned int flags, struct page **pages)
  {
+    const unsigned long nr_pages = (end - addr) >> PAGE_SHIFT;
      struct page *page;
      struct folio *folio;
-    int refs;
+    unsigned long i;
+
+    /* See gup_fast_pte_range() */
+    if (pmd_protnone(orig))
+        return 0;

      if (!pmd_access_permitted(orig, flags & FOLL_WRITE))
          return 0;
@@ -2960,33 +2958,30 @@ static int gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr, 
      if (pmd_special(orig))
          return 0;

-    refs = (end - addr) >> PAGE_SHIFT;
      page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);

-    folio = try_grab_folio_fast(page, refs, flags);
+    folio = try_grab_folio_fast(page, nr_pages, flags);
      if (!folio)
          return 0;

      if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
-        gup_put_folio(folio, refs, flags);
+        gup_put_folio(folio, nr_pages, flags);
          return 0;
      }

      if (!gup_fast_folio_allowed(folio, flags)) {
-        gup_put_folio(folio, refs, flags);
+        gup_put_folio(folio, nr_pages, flags);
          return 0;
      }
     if (!pmd_write(orig) && gup_must_unshare(NULL, flags, &folio- >page)) { 
-        gup_put_folio(folio, refs, flags);
+        gup_put_folio(folio, nr_pages, flags);
          return 0;
      }

-    pages += *nr;
-    *nr += refs;
-    for (; refs; refs--)
+    for (i = 0; i < nr_pages; i++)
          *(pages++) = page++;
      folio_set_referenced(folio);
-    return 1;
+    return nr_pages;
  }

  static int gup_fast_pud_leaf(pud_t orig, pud_t *pudp, unsigned long addr,
@@ -3033,11 +3028,11 @@ static int gup_fast_pud_leaf(pud_t orig, pud_t *pudp, unsigned long addr, 
      return 1;
  }

-static int gup_fast_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr,
-        unsigned long end, unsigned int flags, struct page **pages,
-        int *nr)
+static unsigned long gup_fast_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr, 
+        unsigned long end, unsigned int flags, struct page **pages)
  {
-    unsigned long next;
+    unsigned long cur_nr_pages, next;
+    unsigned long nr_pages = 0;
      pmd_t *pmdp;

      pmdp = pmd_offset_lockless(pudp, pud, addr);
@@ -3046,23 +3041,21 @@ static int gup_fast_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr, 

          next = pmd_addr_end(addr, end);
          if (!pmd_present(pmd))
-            return 0;
+            break;

-        if (unlikely(pmd_leaf(pmd))) {
-            /* See gup_fast_pte_range() */
-            if (pmd_protnone(pmd))
-                return 0;
+        if (unlikely(pmd_leaf(pmd)))
+            cur_nr_pages = gup_fast_pmd_leaf(pmd, pmdp, addr, next, flags, pages); 
+        else
+            cur_nr_pages = gup_fast_pte_range(pmd, pmdp, addr, next, flags, pages); 

-            if (!gup_fast_pmd_leaf(pmd, pmdp, addr, next, flags,
-                pages, nr))
-                return 0;
+        nr_pages += cur_nr_pages;
+        pages += cur_nr_pages;

-        } else if (!gup_fast_pte_range(pmd, pmdp, addr, next, flags,
-                           pages, nr))
-            return 0;
+        if (nr_pages != (next - addr) >> PAGE_SIZE)
+            break;
      } while (pmdp++, addr = next, addr != end);

-    return 1;
+    return nr_pages;
  }

  static int gup_fast_pud_range(p4d_t *p4dp, p4d_t p4d, unsigned long addr,

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