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

David Hildenbrand
2025-09-06T06:57:08Z

On 06.09.25 03:05, John Hubbard wrote:
> On 9/1/25 8:03 AM, David Hildenbrand wrote:
>> We can just cleanup the code by calculating the #refs earlier,
>> so we can just inline what remains of record_subpages().
>>
>> Calculate the number of references/pages ahead of times, and record them
>> only once all our tests passed.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>> mm/gup.c | 25 ++++++++-----------------
>> 1 file changed, 8 insertions(+), 17 deletions(-)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index c10cd969c1a3b..f0f4d1a68e094 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -484,19 +484,6 @@ static inline void mm_set_has_pinned_flag(struct mm_struct *mm)
>> #ifdef CONFIG_MMU
>>
>> #ifdef CONFIG_HAVE_GUP_FAST
>> -static int record_subpages(struct page *page, unsigned long sz,
>> - unsigned long addr, unsigned long end,
>> - struct page **pages)
>> -{
>> - int nr;
>> -
>> - page += (addr & (sz - 1)) >> PAGE_SHIFT;
>> - for (nr = 0; addr != end; nr++, addr += PAGE_SIZE)
>> - pages[nr] = page++;
>> -
>> - return nr;
>> -}
>> -
>> /**
>> * try_grab_folio_fast() - Attempt to get or pin a folio in fast path.
>> * @page: pointer to page to be grabbed
>> @@ -2967,8 +2954,8 @@ static int gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr,
>> if (pmd_special(orig))
>> return 0;
>>
>> - page = pmd_page(orig);
>> - refs = record_subpages(page, PMD_SIZE, addr, end, pages + *nr);
>> + refs = (end - addr) >> PAGE_SHIFT;
>> + page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
>>
>> folio = try_grab_folio_fast(page, refs, flags);
>> if (!folio)
>> @@ -2989,6 +2976,8 @@ static int gup_fast_pmd_leaf(pmd_t orig, pmd_t *pmdp, unsigned long addr,
>> }
>>
>> *nr += refs;
>> + for (; refs; refs--)
>> + *(pages++) = page++;
>> folio_set_referenced(folio);
>> return 1;
>> }
>> @@ -3007,8 +2996,8 @@ static int gup_fast_pud_leaf(pud_t orig, pud_t *pudp, unsigned long addr,
>> if (pud_special(orig))
>> return 0;
>>
>> - page = pud_page(orig);
>> - refs = record_subpages(page, PUD_SIZE, addr, end, pages + *nr);
>> + refs = (end - addr) >> PAGE_SHIFT;
>> + page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
>>
>> folio = try_grab_folio_fast(page, refs, flags);
>> if (!folio)
>> @@ -3030,6 +3019,8 @@ static int gup_fast_pud_leaf(pud_t orig, pud_t *pudp, unsigned long addr,
>> }
>>
>> *nr += refs;
>> + for (; refs; refs--)
>> + *(pages++) = page++;
>
> Hi David,

Hi!

>
> 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".

:)

>
> 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:

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,
--
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)) {
gup_put_folio(folio, 1, flags);
goto pte_unmap;
}
--
2.50.1

--
Cheers

David / dhildenb