On Tue, Jan 26, 2021 at 11:12:14AM -0800, Minchan Kim wrote:

On Tue, Jan 26, 2021 at 08:38:08AM +0100, Michal Hocko wrote:

quoted

On Mon 25-01-21 11:42:34, Minchan Kim wrote:

quoted

On Mon, Jan 25, 2021 at 02:07:01PM +0100, Michal Hocko wrote:

quoted

On Thu 21-01-21 09:54:59, Minchan Kim wrote:

quoted

The upcoming patch will introduce __GFP_NORETRY semantic
in alloc_contig_range which is a failfast mode of the API.
Instead of adding a additional parameter for gfp, replace
no_warn with gfp flag.

To keep old behaviors, it follows the rule below.

  no_warn 			gfp_flags

  false         		GFP_KERNEL
  true          		GFP_KERNEL|__GFP_NOWARN
  gfp & __GFP_NOWARN		GFP_KERNEL | (gfp & __GFP_NOWARN)

Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>

[...]

quoted

diff --git a/mm/cma.c b/mm/cma.c
index 0ba69cd16aeb..d50627686fec 100644
--- a/mm/cma.c
+++ b/mm/cma.c

@@ -419,13 +419,13 @@ static inline void cma_debug_show_areas(struct cma *cma) { }
  * @cma:   Contiguous memory region for which the allocation is performed.
  * @count: Requested number of pages.
  * @align: Requested alignment of pages (in PAGE_SIZE order).
- * @no_warn: Avoid printing message about failed allocation
+ * @gfp_mask: GFP mask to use during the cma allocation.

Call out supported gfp flags explicitly. Have a look at kvmalloc_node
for a guidance.

How about this?

diff --git a/mm/cma.c b/mm/cma.c
index d50627686fec..b94727b694d6 100644
--- a/mm/cma.c
+++ b/mm/cma.c

@@ -423,6 +423,10 @@ static inline void cma_debug_show_areas(struct cma *cma) { }
  *
  * This function allocates part of contiguous memory on specific
  * contiguous memory area.
+ *
+ * For gfp_mask, GFP_KERNEL and __GFP_NORETRY are supported. __GFP_NORETRY
+ * will avoid costly functions(e.g., waiting on page_writeback and locking)
+ * at current implementaion during the page migration.

rather than explicitly mentioning what the flag implies I think it would
be more useful to state the intended usecase. See how kvmalloc_node says
"__GFP_RETRY_MAYFAIL is supported, and it should be used only if kmalloc is
preferable to the vmalloc fallback, due to visible performance
drawbacks.
__GFP_NOWARN is also supported to suppress allocation failure messages."

This would help people not familiar with internals to see whether this
flag is a good fit for them.

In this case I woul go with
"
@flags: gfp mask. Must be compatible (superset) with GFP_KERNEL.
[...]
Reclaim modifiers (__GFP_RETRY_MAYFAIL, __GFP_NOFAIL) are not supported.
__GFP_NORETRY is supported, and it should be used for opportunistic
allocation attempts that should rather fail quickly when the caller has
a fallback strategy.
"

Obviously for this patch you will go with a simple statement that
Reclaim modifiers are not supported at all.

After more discussion for gfp_flags in thread of next patch, let me
changes a bit more based on it.

Thanks for the suggestion, Michal.

Based on the discussion in other thread, I want to go with __GFP_NORETRY
to indicate "opportunistic-easy-to-fail attempt" so suggestion words
Michal is valid so I will carry it on next version.

Thank you.