Re: [RFC 6/7] mm/vmalloc: Support non-blocking GFP flags in __vmalloc_area_node()

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Jul 08, 2025 at 05:47:21PM +0200, Michal Hocko wrote:
> On Fri 04-07-25 17:25:36, Uladzislau Rezki wrote:
> > This patch makes __vmalloc_area_node() to correctly handle non-blocking
> > allocation requests, such as GFP_ATOMIC and GFP_NOWAIT. Main changes:
> > 
> > - nested_gfp flag follows the same non-blocking constraints
> >   as the primary gfp_mask, ensuring consistency and avoiding
> >   sleeping allocations in atomic contexts.
> > 
> > - if blocking is not allowed, __GFP_NOFAIL is forcibly cleared
> >   and warning is issued if it was set, since __GFP_NOFAIL is
> >   incompatible with non-blocking contexts;
> > 
> > - Add a __GFP_HIGHMEM to gfp_mask only for blocking requests
> >   if there are no DMA constraints.
> > 
> > - in non-blocking mode we use memalloc_noreclaim_save/restore()
> >   to prevent reclaim related operations that may sleep while
> >   setting up page tables or mapping pages.
> > 
> > This is particularly important for page table allocations that
> > internally use GFP_PGTABLE_KERNEL, which may sleep unless such
> > scope restrictions are applied. For example:
> > 
> > <snip>
> >     #define GFP_PGTABLE_KERNEL (GFP_KERNEL | __GFP_ZERO)
> > 
> >     __pte_alloc_kernel()
> >         pte_alloc_one_kernel(&init_mm);
> >             pagetable_alloc_noprof(GFP_PGTABLE_KERNEL & ~__GFP_HIGHMEM, 0);
> > <snip>
> > 
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > ---
> >  mm/vmalloc.c | 30 +++++++++++++++++++++++++-----
> >  1 file changed, 25 insertions(+), 5 deletions(-)
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 2eaff0575a9e..fe1699e01e02 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -3711,7 +3711,7 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
> >  				 pgprot_t prot, unsigned int page_shift,
> >  				 int node)
> >  {
> > -	const gfp_t nested_gfp = (gfp_mask & GFP_RECLAIM_MASK) | __GFP_ZERO;
> > +	gfp_t nested_gfp = (gfp_mask & GFP_RECLAIM_MASK) | __GFP_ZERO;
> >  	bool nofail = gfp_mask & __GFP_NOFAIL;
> >  	unsigned long addr = (unsigned long)area->addr;
> >  	unsigned long size = get_vm_area_size(area);
> > @@ -3719,12 +3719,28 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
> >  	unsigned int nr_small_pages = size >> PAGE_SHIFT;
> >  	unsigned int page_order;
> >  	unsigned int flags;
> > +	bool noblock;
> >  	int ret;
> >  
> >  	array_size = (unsigned long)nr_small_pages * sizeof(struct page *);
> > +	noblock = !gfpflags_allow_blocking(gfp_mask);
> >  
> > -	if (!(gfp_mask & (GFP_DMA | GFP_DMA32)))
> > -		gfp_mask |= __GFP_HIGHMEM;
> > +	if (noblock) {
> > +		/* __GFP_NOFAIL is incompatible with non-blocking contexts. */
> > +		WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL);
> > +		gfp_mask &= ~__GFP_NOFAIL;
> 
> Btw. we already ignore GFP_NOFAIL for atomic allocations and warn about
> that at the page allocator level (__alloc_pages_slowpath)
> 
Thank you. I will comment this!

>
> What we can do though is to add a pr_warn + dump_stack for request with
> size that would require (in the worst case) page tables allocation
> larger than a portion of min_free_kbytes (to scale with different memory
> sizes). That should be plenty for any reasonable non blocking vmalloc.
> We would have means to catch abusers in that way.
> 
OK, i will add it. I assume you mean:

  unsigned long pages_min = min_free_kbytes >> (PAGE_SHIFT - 10);

  if (request_pages > pages_min)
    dump();

--
Uladzislau Rezki