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

[Minchan Kim <minchan@kernel.org>]

On Tue, Jan 26, 2021 at 08:38:08AM +0100, Michal Hocko wrote:
> 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@google.com>
> > > > Signed-off-by: Minchan Kim <minchan@kernel.org>
> > > [...]
> > > > 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.