On 03.12.24 16:49, Zi Yan wrote:
On 3 Dec 2024, at 9:24, Vlastimil Babka wrote:
On 12/3/24 15:12, David Hildenbrand wrote:
On 03.12.24 14:55, Vlastimil Babka wrote:
On 12/3/24 10:47, David Hildenbrand wrote:
It's all a bit complicated for alloc_contig_range(). For example, we don't
support many flags, so let's start bailing out on unsupported
ones -- ignoring the placement hints, as we are already given the range
to allocate.
While we currently set cc.gfp_mask, in __alloc_contig_migrate_range() we
simply create yet another GFP mask whereby we ignore the reclaim flags
specify by the caller. That looks very inconsistent.
Let's clean it up, constructing the gfp flags used for
compaction/migration exactly once. Update the documentation of the
gfp_mask parameter for alloc_contig_range() and alloc_contig_pages().
Acked-by: Zi Yan <ziy@xxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
+ /*
+ * Flags to control page compaction/migration/reclaim, to free up our
+ * page range. Migratable pages are movable, __GFP_MOVABLE is implied
+ * for them.
+ *
+ * Traditionally we always had __GFP_HARDWALL|__GFP_RETRY_MAYFAIL set,
+ * keep doing that to not degrade callers.
+ */
Wonder if we could revisit that eventually. Why limit migration targets by
cpuset via __GFP_HARDWALL if we were not called with __GFP_HARDWALL? And why
weaken the attempts with __GFP_RETRY_MAYFAIL if we didn't specify it?
See below.
Unless I'm missing something, cc->gfp is only checked for __GFP_FS and
__GFP_NOWARN in few places, so it's mostly migration_target_control the
callers could meaningfully influence.
Note the fist change in the file, where we now use the mask instead of coming up
with another one out of the blue. :)
I know. What I wanted to say - cc->gfp is on its own only checked in few
places, but now since we also translate it to migration_target_control's
gfp_mask, it's mostly that part the caller might influence with the passed
flags. But we still impose own additions to it, limiting that influence.
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ce7589a4ec01..54594cc4f650 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6294,7 +6294,7 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
int ret = 0;
struct migration_target_control mtc = {
.nid = zone_to_nid(cc->zone),
- .gfp_mask = GFP_USER | __GFP_MOVABLE | __GFP_RETRY_MAYFAIL,
+ .gfp_mask = cc->gfp_mask,
.reason = MR_CONTIG_RANGE,
};
GFP_USER contains __GFP_HARDWALL. I am not sure if that matters here, but
Yeah wonder if GFP_USER was used specifically for that part, or just randomly :)
likely the thing we are assuming here is that we are migrating a page, and
usually, these are user allocation (except maybe balloon and some other non-lru
movable things).
Yeah and user allocations obey cpuset and mempolicies etc. But these are
likely somebody elses allocations that were done according to their
policies. With our migration we might be actually violating those, which
probably can't be helped (is at least migration within the same node
preferred? hmm). But it doesn't seem to me that our caller's restrictions
(if those exist, would be enforced by __GFP_HARDWALL) are that relevant for
somebody else's pages?
Yeah, I was wondering why current_gfp_context() is used to adjust gfp_mask,
since current context might not be relevant. But I see it is used in
the original code, so I did not ask. If current context is irrelevant w.r.t
the to-be-migrated pages, should current_gfp_context() part be removed?
Please see how current_gfp_context() is only concerned (excluding the
__GFP_MOVABLE thing we unconditionally set ...) about reclaim flags.
This part make absolute sense to respect here.
So that is something different than __GFP_HARDWALL that *we so far
unconditionally set* and is not a "reclaim" flag.
--
Cheers,
David / dhildenb
