Re: [PATCH RESEND v2 4/6] mm/page_alloc: sort out the alloc_contig_range() gfp flags mess

[David Hildenbrand <david@redhat.com>]

On 04.12.24 10:15, Oscar Salvador wrote:
> On Wed, Dec 04, 2024 at 10:03:28AM +0100, Vlastimil Babka wrote:
>> On 12/4/24 09:59, Oscar Salvador wrote:
>>> On Tue, Dec 03, 2024 at 08:19:02PM +0100, David Hildenbrand wrote:
>>>> It was always set using "GFP_USER | __GFP_MOVABLE | __GFP_RETRY_MAYFAIL",
>>>> and I removed the same flag combination in #2 from memory offline code, and
>>>> we do have the exact same thing in do_migrate_range() in
>>>> mm/memory_hotplug.c.
>>>>
>>>> We should investigate if__GFP_HARDWALL is the right thing to use here, and
>>>> if we can get rid of that by switching to GFP_KERNEL in all these places.
>>>
>>> Why would not we want __GFP_HARDWALL set?
>>> Without it, we could potentially migrate the page to a node which is not
>>> part of the cpuset of the task that originally allocated it, thus violating the
>>> policy? Is not that a problem?
>>
>> The task doing the alloc_contig_range() will likely not be the same task as
>> the one that originally allocated the page, so its policy would be
>> different, no? So even with __GFP_HARDWALL we might be already migrating
>> outside the original tasks's constraint? Am I missing something?
> 
> Yes, that is right, I thought we derive the policy from the old page
> somehow when migrating it, but reading the code does not seem to be the
> case.
> 
> Looking at prepare_alloc_pages(), if !ac->nodemask, which would be the
> case here, we would get the policy from the current task
> (alloc_contig_range()) when cpusets are enabled.
> 
> So yes, I am a bit puzzled why __GFP_HARDWALL was chosen in the first
> place.

I suspect because "GFP_USER" felt like the appropriate thing to do.

Before:

commit f90b1d2f1aaaa40c6519a32e69615edc25bb97d5
Author: Paul Jackson <pj@sgi.com>
Date:   Tue Sep 6 15:18:10 2005 -0700

     [PATCH] cpusets: new __GFP_HARDWALL flag

     Add another GFP flag: __GFP_HARDWALL.


GFP_USER  and GFP_KERNEL were the same thing. But memory 
offlining/alloc_contig were added later.

-- 
Cheers,

David / dhildenb