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

[David Hildenbrand <david@redhat.com>]

On 08.09.25 14:25, Lorenzo Stoakes wrote:
> 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...!

To be clear: it was never intended to be applied, because it wouldn't 
even compile in the current form.

It was based on this nth_page submission + fix.


[...]

>>   }
>>   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.

Yes. Expect a cleanup in that direction soonish (again, either from me 
or someone else I poke)

> 
>> --
>> 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...

I sent it out earlier today. Fortunately that function shouldn't usually 
really fail IIUC.

-- 
Cheers

David / dhildenb