Re: [PATCH 7/8] mm/vmalloc: Support non-blocking GFP flags in __vmalloc_area_node()

[Baoquan He <bhe@redhat.com>]

On 08/18/25 at 03:08pm, Uladzislau Rezki wrote:
> On Mon, Aug 18, 2025 at 12:35:16PM +0800, Baoquan He wrote:
> > On 08/07/25 at 09:58am, Uladzislau Rezki (Sony) wrote:
> > > This patch makes __vmalloc_area_node() to correctly handle non-blocking
> > > allocation requests, such as GFP_ATOMIC and GFP_NOWAIT. Main changes:
> > > 
> > > - Add a __GFP_HIGHMEM to gfp_mask only for blocking requests
> > >   if there are no DMA constraints.
> > > 
> > > - vmap_page_range() is wrapped by memalloc_noreclaim_save/restore()
> > >   to avoid memory reclaim related operations that could sleep during
> > >   page table setup 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>
> > > __pte_alloc_kernel()
> > >     pte_alloc_one_kernel(&init_mm);
> > >         pagetable_alloc_noprof(GFP_PGTABLE_KERNEL & ~__GFP_HIGHMEM, 0);
> > > <snip>
> > > 
> > > Note: in most cases, PTE entries are established only up to the level
> > > required by current vmap space usage, meaning the page tables are typically
> > > fully populated during the mapping process.
> > > 
> > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > > ---
> > >  mm/vmalloc.c | 20 ++++++++++++++++----
> > >  1 file changed, 16 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > index 2424f80d524a..8a7eab810561 100644
> > > --- a/mm/vmalloc.c
> > > +++ b/mm/vmalloc.c
> > > @@ -3721,12 +3721,20 @@ 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 and "noblock" flags are mutually exclusive. */
> > > +		nofail = false;
> > > +	} else {
> > > +		/* Allow highmem allocations if there are no DMA constraints. */
> > > +		if (!(gfp_mask & (GFP_DMA | GFP_DMA32)))
> > > +			gfp_mask |= __GFP_HIGHMEM;
> > > +	}
> > >  
> > >  	/* Please note that the recursion is strictly bounded. */
> > >  	if (array_size > PAGE_SIZE) {
> > > @@ -3790,7 +3798,9 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
> > >  	 * page tables allocations ignore external gfp mask, enforce it
> > >  	 * by the scope API
> > >  	 */
> > > -	if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
> > > +	if (noblock)
> > > +		flags = memalloc_noreclaim_save();
> > > +	else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
> > >  		flags = memalloc_nofs_save();
> > >  	else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == 0)
> > >  		flags = memalloc_noio_save();
> > > @@ -3802,7 +3812,9 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
> > >  			schedule_timeout_uninterruptible(1);
> > >  	} while (nofail && (ret < 0));
> > >  
> > > -	if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
> > > +	if (noblock)
> > > +		memalloc_noreclaim_restore(flags);
> > > +	else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
> > >  		memalloc_nofs_restore(flags);
> > >  	else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == 0)
> > >  		memalloc_noio_restore(flags);
> > 
> > Can we use memalloc_flags_restore(flags) directly to replace above if
> > else checking? It can reduce LOC, might be not as readable as the change
> > in patch surely. Not strong opinion.
> > 
> > 	memalloc_flags_restore(flags);
> > 
> I agree, those if/else cases looks ugly. Maybe adding two save/restore
> functions are worth doing specifically for vmalloc part.

Yeah, that is also great idea.