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

On Sat, Sep 06, 2025 at 08:56:48AM +0200, David Hildenbrand wrote:

On 06.09.25 03:05, John Hubbard wrote:

...

Probably a similar sentiment as Lorenzo here...the above diffs make the code *worse* to read. In fact, I recall adding record_subpages() here long ago, specifically to help clarify what was going on.

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".

Yes

:)

...

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.

Roughly, what I am thinking (limiting it to pte+pmd case) about is the following:

I cannot get the below to apply even with the original patch here applied + fix.

It looks like (in mm-new :) commit e73f43a66d5f ("mm/gup: remove dead pgmap refcounting code") by Alastair has conflicted here, but even then I can't make it apply, with/without your fix...!

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

Signed-off-by: David Hildenbrand david@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,

OK I guess you intentionally left the rest as a TODO :)

So I'll wait for you to post it before reviewing in-depth.

This generally LGTM as an approach, getting rid of *nr is important that's really horrible.

-- 2.50.1

Oh, I might even have found a bug moving away from that questionable "ret==1 means success" handling in gup_fast_pte_range()? Will have to double-check, but likely the following is the right thing to do.

From 8f48b25ef93e7ef98611fd58ec89384ad5171782 Mon Sep 17 00:00:00 2001 From: David Hildenbrand david@redhat.com Date: Sat, 6 Sep 2025 08:46:45 +0200 Subject: [PATCH] mm/gup: fix handling of errors from arch_make_folio_accessible() in follow_page_pte()

In case we call arch_make_folio_accessible() and it fails, we would incorrectly return a value that is "!= 0" to the caller, indicating that we pinned all requested pages and that the caller can keep going.

follow_page_pte() is not supposed to return error values, but instead 0 on failure and 1 on success.

That is of course wrong, because the caller will just keep going pinning more pages. If we happen to pin a page afterwards, we're in trouble, because we essentially skipped some pages.

Fixes: f28d43636d6f ("mm/gup/writeback: add callbacks for inaccessible pages") Signed-off-by: David Hildenbrand david@redhat.com

---

mm/gup.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c index 22420f2069ee1..cff226ec0ee7d 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -2908,8 +2908,7 @@ 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)) {
  

Oh Lord above. Lol. Yikes.

Yeah I think your fix is valid...

		gup_put_folio(folio, 1, flags);
		goto pte_unmap;
	}

-- 2.50.1

-- Cheers

David / dhildenb