On 04.12.24 11:04, Oscar Salvador wrote:
On Wed, Dec 04, 2024 at 10:28:39AM +0100, David Hildenbrand wrote:
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.
Looking back at when the whole contiguous allocator patchset was posted,
it seems that it kinda copied what memory-offline code was doing, which
was migrating pages with GFP_HIGHUSER_MOVABLE (hotremove_migrate_alloc()).
Then, the HIGHMEM modifier was dropped due to HIGHMEM restrictions on
some systems, ending up with GFP_USER.
Looking at some other migration_target_control users, some of them also
shouldn't be setting GFP_USER->HARDWALL either I think. Essentially,
whenever we are migrating a page that is not guaranteed to be "ours" in
the context of the caller.
mm/damon/paddr.c:__damon_pa_migrate_folio_list() for example, which
obtained the addresses by scanning a chunk of physical address space.
For others it's less clear: soft_offline_in_use_page() may be called
either using madvise() from process context, but also using sysfs using
a PFN.
--
Cheers,
David / dhildenb
