Re: [PATCH V6] mm, compaction: don't use ALLOC_CMA in long term GUP flow

Ge Yang <yangge1116@xxxxxxx> · Tue, 17 Dec 2024 17:12:23 +0800

在 2024/12/17 16:49, Barry Song 写道:
On Tue, Dec 17, 2024 at 8:12 PM Ge Yang <yangge1116@xxxxxxx> wrote:



在 2024/12/17 14:14, Barry Song 写道:
On Tue, Dec 17, 2024 at 4:33 PM <yangge1116@xxxxxxx> wrote:

From: yangge <yangge1116@xxxxxxx>

Since commit 984fdba6a32e ("mm, compaction: use proper alloc_flags
in __compaction_suitable()") allow compaction to proceed when free
pages required for compaction reside in the CMA pageblocks, it's
possible that __compaction_suitable() always returns true, and in
some cases, it's not acceptable.

There are 4 NUMA nodes on my machine, and each NUMA node has 32GB
of memory. I have configured 16GB of CMA memory on each NUMA node,
and starting a 32GB virtual machine with device passthrough is
extremely slow, taking almost an hour.

During the start-up of the virtual machine, it will call
pin_user_pages_remote(..., FOLL_LONGTERM, ...) to allocate memory.
Long term GUP cannot allocate memory from CMA area, so a maximum
of 16 GB of no-CMA memory on a NUMA node can be used as virtual
machine memory. Since there is 16G of free CMA memory on the NUMA

Other unmovable allocations, like dma_buf, which can be large in a
Linux system, are
also unable to allocate memory from CMA. My question is whether the issue you
described applies to these allocations as well.
pin_user_pages_remote(..., FOLL_LONGTERM, ...) first attemps to allocate
THP only on local node, and then fall back to remote NUMA nodes if the
local allocation fail. For detail, see alloc_pages_mpol().

static struct page *alloc_pages_mpol()
{
      page = __alloc_frozen_pages_noprof(__GFP_THISNODE,...); // 1, try
to allocate THP only on local node

      if (page || !(gpf & __GFP_DIRECT_RECLAIM))
          return page;

      page = __alloc_frozen_pages_noprof(gfp, order, nid, nodemask);//2,
fall back to remote NUMA nodes
}

If dma_buf also uses the same way to allocate memory，dma_buf will also
have this problem.

Imagine we have only one NUMA node and no remote nodes to 'borrow'
memory from. What would happen as a result of this bug?

__alloc_pages_slowpath() can't exit at the appropriate
place, it will continue to perform meaningless memory reclaim and 
compaction.




node, watermark for order-0 always be met for compaction, so
__compaction_suitable() always returns true, even if the node is
unable to allocate non-CMA memory for the virtual machine.

For costly allocations, because __compaction_suitable() always
returns true, __alloc_pages_slowpath() can't exit at the appropriate
place, resulting in excessively long virtual machine startup times.
Call trace:
__alloc_pages_slowpath
      if (compact_result == COMPACT_SKIPPED ||
          compact_result == COMPACT_DEFERRED)
          goto nopage; // should exit __alloc_pages_slowpath() from here


Do we face the same issue if we allocate dma-buf while CMA has plenty
of free memory, but non-CMA has none?

In order to quickly fall back to remote node, we should remove
ALLOC_CMA both in __compaction_suitable() and __isolate_free_page()
in long term GUP flow. After this fix, starting a 32GB virtual machine
with device passthrough takes only a few seconds.

Fixes: 984fdba6a32e ("mm, compaction: use proper alloc_flags in __compaction_suitable()")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: yangge <yangge1116@xxxxxxx>
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---

V6:
-- update cc->alloc_flags to keep the original loginc

V5:
- add 'alloc_flags' parameter for __isolate_free_page()
- remove 'usa_cma' variable

V4:
- rich the commit log description

V3:
- fix build errors
- add ALLOC_CMA both in should_continue_reclaim() and compaction_ready()

V2:
- using the 'cc->alloc_flags' to determin if 'ALLOC_CMA' is needed
- rich the commit log description

   include/linux/compaction.h |  6 ++++--
   mm/compaction.c            | 26 +++++++++++++++-----------
   mm/internal.h              |  3 ++-
   mm/page_alloc.c            |  7 +++++--
   mm/page_isolation.c        |  3 ++-
   mm/page_reporting.c        |  2 +-
   mm/vmscan.c                |  4 ++--
   7 files changed, 31 insertions(+), 20 deletions(-)

diff --git a/include/linux/compaction.h b/include/linux/compaction.h
index e947764..b4c3ac3 100644
--- a/include/linux/compaction.h
+++ b/include/linux/compaction.h
@@ -90,7 +90,8 @@ extern enum compact_result try_to_compact_pages(gfp_t gfp_mask,
                  struct page **page);
   extern void reset_isolation_suitable(pg_data_t *pgdat);
   extern bool compaction_suitable(struct zone *zone, int order,
-                                              int highest_zoneidx);
+                                              int highest_zoneidx,
+                                              unsigned int alloc_flags);

   extern void compaction_defer_reset(struct zone *zone, int order,
                                  bool alloc_success);
@@ -108,7 +109,8 @@ static inline void reset_isolation_suitable(pg_data_t *pgdat)
   }

   static inline bool compaction_suitable(struct zone *zone, int order,
-                                                     int highest_zoneidx)
+                                                     int highest_zoneidx,
+                                                     unsigned int alloc_flags)
   {
          return false;
   }
diff --git a/mm/compaction.c b/mm/compaction.c
index 07bd227..d92ba6c 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -655,7 +655,7 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,

                  /* Found a free page, will break it into order-0 pages */
                  order = buddy_order(page);
-               isolated = __isolate_free_page(page, order);
+               isolated = __isolate_free_page(page, order, cc->alloc_flags);
                  if (!isolated)
                          break;
                  set_page_private(page, order);
@@ -1634,7 +1634,7 @@ static void fast_isolate_freepages(struct compact_control *cc)

                  /* Isolate the page if available */
                  if (page) {
-                       if (__isolate_free_page(page, order)) {
+                       if (__isolate_free_page(page, order, cc->alloc_flags)) {
                                  set_page_private(page, order);
                                  nr_isolated = 1 << order;
                                  nr_scanned += nr_isolated - 1;
@@ -2381,6 +2381,7 @@ static enum compact_result compact_finished(struct compact_control *cc)

   static bool __compaction_suitable(struct zone *zone, int order,
                                    int highest_zoneidx,
+                                 unsigned int alloc_flags,
                                    unsigned long wmark_target)
   {
          unsigned long watermark;
@@ -2395,25 +2396,26 @@ static bool __compaction_suitable(struct zone *zone, int order,
           * even if compaction succeeds.
           * For costly orders, we require low watermark instead of min for
           * compaction to proceed to increase its chances.
-        * ALLOC_CMA is used, as pages in CMA pageblocks are considered
-        * suitable migration targets
+        * In addition to long term GUP flow, ALLOC_CMA is used, as pages in
+        * CMA pageblocks are considered suitable migration targets

I'm not sure if this document is correct for cases other than GUP.

           */
          watermark = (order > PAGE_ALLOC_COSTLY_ORDER) ?
                                  low_wmark_pages(zone) : min_wmark_pages(zone);
          watermark += compact_gap(order);
          return __zone_watermark_ok(zone, 0, watermark, highest_zoneidx,
-                                  ALLOC_CMA, wmark_target);
+                                  alloc_flags & ALLOC_CMA, wmark_target);
   }

   /*
    * compaction_suitable: Is this suitable to run compaction on this zone now?
    */
-bool compaction_suitable(struct zone *zone, int order, int highest_zoneidx)
+bool compaction_suitable(struct zone *zone, int order, int highest_zoneidx,
+                                  unsigned int alloc_flags)
   {
          enum compact_result compact_result;
          bool suitable;

-       suitable = __compaction_suitable(zone, order, highest_zoneidx,
+       suitable = __compaction_suitable(zone, order, highest_zoneidx, alloc_flags,
                                           zone_page_state(zone, NR_FREE_PAGES));
          /*
           * fragmentation index determines if allocation failures are due to
@@ -2474,7 +2476,7 @@ bool compaction_zonelist_suitable(struct alloc_context *ac, int order,
                  available = zone_reclaimable_pages(zone) / order;
                  available += zone_page_state_snapshot(zone, NR_FREE_PAGES);
                  if (__compaction_suitable(zone, order, ac->highest_zoneidx,
-                                         available))
+                                         alloc_flags, available))
                          return true;
          }

@@ -2499,7 +2501,7 @@ compaction_suit_allocation_order(struct zone *zone, unsigned int order,
                                alloc_flags))
                  return COMPACT_SUCCESS;

-       if (!compaction_suitable(zone, order, highest_zoneidx))
+       if (!compaction_suitable(zone, order, highest_zoneidx, alloc_flags))
                  return COMPACT_SKIPPED;

          return COMPACT_CONTINUE;
@@ -2893,6 +2895,7 @@ static int compact_node(pg_data_t *pgdat, bool proactive)
          struct compact_control cc = {
                  .order = -1,
                  .mode = proactive ? MIGRATE_SYNC_LIGHT : MIGRATE_SYNC,
+               .alloc_flags = ALLOC_CMA,
                  .ignore_skip_hint = true,
                  .whole_zone = true,
                  .gfp_mask = GFP_KERNEL,
@@ -3037,7 +3040,7 @@ static bool kcompactd_node_suitable(pg_data_t *pgdat)

                  ret = compaction_suit_allocation_order(zone,
                                  pgdat->kcompactd_max_order,
-                               highest_zoneidx, ALLOC_WMARK_MIN);
+                               highest_zoneidx, ALLOC_CMA | ALLOC_WMARK_MIN);
                  if (ret == COMPACT_CONTINUE)
                          return true;
          }
@@ -3058,6 +3061,7 @@ static void kcompactd_do_work(pg_data_t *pgdat)
                  .search_order = pgdat->kcompactd_max_order,
                  .highest_zoneidx = pgdat->kcompactd_highest_zoneidx,
                  .mode = MIGRATE_SYNC_LIGHT,
+               .alloc_flags = ALLOC_CMA | ALLOC_WMARK_MIN,
                  .ignore_skip_hint = false,
                  .gfp_mask = GFP_KERNEL,
          };
@@ -3078,7 +3082,7 @@ static void kcompactd_do_work(pg_data_t *pgdat)
                          continue;

                  ret = compaction_suit_allocation_order(zone,
-                               cc.order, zoneid, ALLOC_WMARK_MIN);
+                               cc.order, zoneid, cc.alloc_flags);
                  if (ret != COMPACT_CONTINUE)
                          continue;

diff --git a/mm/internal.h b/mm/internal.h
index 3922788..6d257c8 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -662,7 +662,8 @@ static inline void clear_zone_contiguous(struct zone *zone)
          zone->contiguous = false;
   }

-extern int __isolate_free_page(struct page *page, unsigned int order);
+extern int __isolate_free_page(struct page *page, unsigned int order,
+                                   unsigned int alloc_flags);
   extern void __putback_isolated_page(struct page *page, unsigned int order,
                                      int mt);
   extern void memblock_free_pages(struct page *page, unsigned long pfn,
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index dde19db..1bfdca3 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2809,7 +2809,8 @@ void split_page(struct page *page, unsigned int order)
   }
   EXPORT_SYMBOL_GPL(split_page);

-int __isolate_free_page(struct page *page, unsigned int order)
+int __isolate_free_page(struct page *page, unsigned int order,
+                                  unsigned int alloc_flags)
   {
          struct zone *zone = page_zone(page);
          int mt = get_pageblock_migratetype(page);
@@ -2823,7 +2824,8 @@ int __isolate_free_page(struct page *page, unsigned int order)
                   * exists.
                   */
                  watermark = zone->_watermark[WMARK_MIN] + (1UL << order);
-               if (!zone_watermark_ok(zone, 0, watermark, 0, ALLOC_CMA))
+               if (!zone_watermark_ok(zone, 0, watermark, 0,
+                           alloc_flags & ALLOC_CMA))
                          return 0;
          }

@@ -6454,6 +6456,7 @@ int alloc_contig_range_noprof(unsigned long start, unsigned long end,
                  .order = -1,
                  .zone = page_zone(pfn_to_page(start)),
                  .mode = MIGRATE_SYNC,
+               .alloc_flags = ALLOC_CMA,
                  .ignore_skip_hint = true,
                  .no_set_skip_hint = true,
                  .alloc_contig = true,
diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index c608e9d..a1f2c79 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -229,7 +229,8 @@ static void unset_migratetype_isolate(struct page *page, int migratetype)
                          buddy = find_buddy_page_pfn(page, page_to_pfn(page),
                                                      order, NULL);
                          if (buddy && !is_migrate_isolate_page(buddy)) {
-                               isolated_page = !!__isolate_free_page(page, order);
+                               isolated_page = !!__isolate_free_page(page, order,
+                                                   ALLOC_CMA);
                                  /*
                                   * Isolating a free page in an isolated pageblock
                                   * is expected to always work as watermarks don't
diff --git a/mm/page_reporting.c b/mm/page_reporting.c
index e4c428e..fd3813b 100644
--- a/mm/page_reporting.c
+++ b/mm/page_reporting.c
@@ -198,7 +198,7 @@ page_reporting_cycle(struct page_reporting_dev_info *prdev, struct zone *zone,

                  /* Attempt to pull page from list and place in scatterlist */
                  if (*offset) {
-                       if (!__isolate_free_page(page, order)) {
+                       if (!__isolate_free_page(page, order, ALLOC_CMA)) {
                                  next = page;
                                  break;
                          }
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 5e03a61..33f5b46 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -5815,7 +5815,7 @@ static inline bool should_continue_reclaim(struct pglist_data *pgdat,
                                        sc->reclaim_idx, 0))
                          return false;

-               if (compaction_suitable(zone, sc->order, sc->reclaim_idx))
+               if (compaction_suitable(zone, sc->order, sc->reclaim_idx, ALLOC_CMA))
                          return false;
          }

@@ -6043,7 +6043,7 @@ static inline bool compaction_ready(struct zone *zone, struct scan_control *sc)
                  return true;

          /* Compaction cannot yet proceed. Do reclaim. */
-       if (!compaction_suitable(zone, sc->order, sc->reclaim_idx))
+       if (!compaction_suitable(zone, sc->order, sc->reclaim_idx, ALLOC_CMA))
                  return false;

          /*
--
2.7.4



Thanks
Barry










