Re: [PATCH] mm: allow __GFP_RETRY_MAYFAIL in vmalloc

[Michal Hocko <mhocko@suse.com>]

On Tue 24-02-26 16:44:43, Michal Hocko wrote:
> On Tue 24-02-26 16:38:01, Uladzislau Rezki wrote:
[...]
> > I do not have a strong opinion about workaround you noted. Maybe Mikulas
> > can switch to NOWAIT flag instead.
> 
> using NOWAIT for the full vmalloc allocation would be just too easy to
> fail under moderate memory pressure.
> 
> The real question is whether we want to provide some sort of backoff
> early but not way too easily allocation semantic for vmalloc. If yes we
> need to get creative in the vmalloc internals rather than expect callers
> to be working around that on their side. History has proven that this
> just leads to tech. dept and more work later on.

Just to make sure we are on the same page I mean something like this 
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 61caa55a4402..791366fe44e2 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3798,6 +3798,8 @@ static void defer_vm_area_cleanup(struct vm_struct *area)
  * non-blocking (no __GFP_DIRECT_RECLAIM) - memalloc_noreclaim_save()
  * GFP_NOFS - memalloc_nofs_save()
  * GFP_NOIO - memalloc_noio_save()
+ * __GFP_RETRY_MAYFAIL, __GFP_NORETRY - memalloc_noreclaim_save() to prevent
+ * OOMs
  *
  * Returns a flag cookie to pair with restore.
  */
@@ -3806,7 +3808,7 @@ memalloc_apply_gfp_scope(gfp_t gfp_mask)
 {
 	unsigned int flags = 0;
 
-	if (!gfpflags_allow_blocking(gfp_mask))
+	if (!gfpflags_allow_blocking(gfp_mask) || (gfp_mask & (__GFP_RETRY_MAYFAIL|__GFP_NORETRY)))
 		flags = memalloc_noreclaim_save();
 	else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
 		flags = memalloc_nofs_save();
@@ -3940,7 +3942,8 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
  * GFP_KERNEL_ACCOUNT. Xfs uses __GFP_NOLOCKDEP.
  */
 #define GFP_VMALLOC_SUPPORTED (GFP_KERNEL | GFP_ATOMIC | GFP_NOWAIT |\
-				__GFP_NOFAIL |  __GFP_ZERO | __GFP_NORETRY |\
+				__GFP_NOFAIL |  __GFP_ZERO | |\
+				__GFP_NORETRY | __GFP_RETRY_MAYFAIL |\
 				GFP_NOFS | GFP_NOIO | GFP_KERNEL_ACCOUNT |\
 				GFP_USER | __GFP_NOLOCKDEP)
 
@@ -3971,12 +3974,15 @@ static gfp_t vmalloc_fix_flags(gfp_t flags)
  * virtual range with protection @prot.
  *
  * Supported GFP classes: %GFP_KERNEL, %GFP_ATOMIC, %GFP_NOWAIT,
- * %GFP_NOFS and %GFP_NOIO. Zone modifiers are not supported.
+ * %__GFP_RETRY_MAYFAIL, %__GFP_NORETRY, %GFP_NOFS and %GFP_NOIO.
+ * Zone modifiers are not supported.
  * Please note %GFP_ATOMIC and %GFP_NOWAIT are supported only
  * by __vmalloc().
  *
- * Retry modifiers: only %__GFP_NOFAIL is supported; %__GFP_NORETRY
- * and %__GFP_RETRY_MAYFAIL are not supported.
+ * Retry modifiers: only %__GFP_NOFAIL is fully supported; 
+ * %__GFP_NORETRY and %__GFP_RETRY_MAYFAIL are supported with limitation,
+ * i.e. page tables are allocated with NOWAIT semantic so they might fail
+ * under moderate memory pressure. 
  *
  * %__GFP_NOWARN can be used to suppress failure messages.
  *

-- 
Michal Hocko
SUSE Labs