Re: [PATCH v4 1/4] mm: cma: introduce gfp flag in cma_alloc instead of no_warn

[Michal Hocko]

On Mon 25-01-21 11:42:34, Minchan Kim wrote:
> On Mon, Jan 25, 2021 at 02:07:01PM +0100, Michal Hocko wrote:
> > On Thu 21-01-21 09:54:59, Minchan Kim wrote:
> > > 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@xxxxxxxxxx>
> > > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> > [...]
> > > 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.

>   */
>  struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
>                        gfp_t gfp_mask)
> 

-- 
Michal Hocko
SUSE Labs