Re: [PATCH 3/4] mm/vmalloc: be more explicit about supported gfp flags.

[Michal Hocko <mhocko@suse.com>]

On Tue 26-10-21 21:43:17, Neil Brown wrote:
> On Tue, 26 Oct 2021, Michal Hocko wrote:
> > On Tue 26-10-21 10:26:06, Neil Brown wrote:
> > > On Tue, 26 Oct 2021, Michal Hocko wrote:
> > > > From: Michal Hocko <mhocko@suse.com>
> > > > 
> > > > The core of the vmalloc allocator __vmalloc_area_node doesn't say
> > > > anything about gfp mask argument. Not all gfp flags are supported
> > > > though. Be more explicit about constrains.
> > > > 
> > > > Signed-off-by: Michal Hocko <mhocko@suse.com>
> > > > ---
> > > >  mm/vmalloc.c | 12 ++++++++++--
> > > >  1 file changed, 10 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > > index 602649919a9d..2199d821c981 100644
> > > > --- a/mm/vmalloc.c
> > > > +++ b/mm/vmalloc.c
> > > > @@ -2980,8 +2980,16 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
> > > >   * @caller:		  caller's return address
> > > >   *
> > > >   * Allocate enough pages to cover @size from the page level
> > > > - * allocator with @gfp_mask flags.  Map them into contiguous
> > > > - * kernel virtual space, using a pagetable protection of @prot.
> > > > + * allocator with @gfp_mask flags. Please note that the full set of gfp
> > > > + * flags are not supported. GFP_KERNEL would be a preferred allocation mode
> > > > + * but GFP_NOFS and GFP_NOIO are supported as well. Zone modifiers are not
> > > 
> > > In what sense is GFP_KERNEL "preferred"??
> > > The choice of GFP_NOFS, when necessary, isn't based on preference but
> > > on need.
> > > 
> > > I understand that you would prefer no one ever used GFP_NOFs ever - just
> > > use the scope API.  I even agree.  But this is not the place to make
> > > that case. 
> > 
> > Any suggestion for a better wording?
> 
>  "GFP_KERNEL, GFP_NOFS, and GFP_NOIO are all supported".

OK. Check the incremental update at the end of the email

> > > > + * supported. From the reclaim modifiers__GFP_DIRECT_RECLAIM is required (aka
> > > > + * GFP_NOWAIT is not supported) and only __GFP_NOFAIL is supported (aka
> > > 
> > > I don't think "aka" is the right thing to use here.  It is short for
> > > "also known as" and there is nothing that is being known as something
> > > else.
> > > It would be appropriate to say (i.e. GFP_NOWAIT is not supported).
> > > "i.e." is short for the Latin "id est" which means "that is" and
> > > normally introduces an alternate description (whereas aka introduces an
> > > alternate name).
> > 
> > OK
> >  
> > > > + * __GFP_NORETRY and __GFP_RETRY_MAYFAIL are not supported).
> > > 
> > > Why do you think __GFP_NORETRY and __GFP_RETRY_MAYFAIL are not supported.
> > 
> > Because they cannot be passed to the page table allocator. In both cases
> > the allocation would fail when system is short on memory. GFP_KERNEL
> > used for ptes implicitly doesn't behave that way.
> 
> Could you please point me to the particular allocation which uses
> GFP_KERNEL rather than the flags passed to __vmalloc_node()?  I cannot
> find it.
> 

It is dug 
__vmalloc_area_node
  vmap_pages_range
    vmap_pages_range_noflush
      vmap_range_noflush || vmap_small_pages_range_noflush
        vmap_p4d_range
	  p4d_alloc_track
	    __p4d_alloc
	      p4d_alloc_one
	        get_zeroed_page(GFP_KERNEL_ACCOUNT)

the same applies for all other levels of page tables.

This is what I have currently
commit ae7fc6c2ef6949a76d697fc61bb350197dfca330
Author: Michal Hocko <mhocko@suse.com>
Date:   Tue Oct 26 14:16:32 2021 +0200

    fold me "mm/vmalloc: be more explicit about supported gfp flags."

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 2ddaa9410aee..82a07b04317e 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2981,12 +2981,14 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
  *
  * Allocate enough pages to cover @size from the page level
  * allocator with @gfp_mask flags. Please note that the full set of gfp
- * flags are not supported. GFP_KERNEL would be a preferred allocation mode
- * but GFP_NOFS and GFP_NOIO are supported as well. Zone modifiers are not
- * supported. From the reclaim modifiers__GFP_DIRECT_RECLAIM is required (aka
- * GFP_NOWAIT is not supported) and only __GFP_NOFAIL is supported (aka
- * __GFP_NORETRY and __GFP_RETRY_MAYFAIL are not supported).
- * __GFP_NOWARN can be used to suppress error messages about failures.
+ * flags are not supported. GFP_KERNEL, GFP_NOFS, and GFP_NOIO are all
+ * supported.
+ * Zone modifiers are not supported. From the reclaim modifiers
+ * __GFP_DIRECT_RECLAIM is required (aka GFP_NOWAIT is not supported)
+ * and only __GFP_NOFAIL is supported (i.e. __GFP_NORETRY and 
+ * __GFP_RETRY_MAYFAIL are not supported).
+ *
+ * __GFP_NOWARN can be used to suppress failures messages.
  * 
  * Map them into contiguous kernel virtual space, using a pagetable
  * protection of @prot.
-- 
Michal Hocko
SUSE Labs