Re: [RESEND PATCH v1] mm/vmalloc: fix page mapping if vm_area_alloc_pages() with high order fallback to order 0

Uladzislau Rezki
2024-08-26T12:38:50Z

On Mon, Aug 26, 2024 at 09:52:42AM +0200, Michal Hocko wrote:
> On Fri 23-08-24 18:42:47, Uladzislau Rezki wrote:
> [...]
> > @@ -3666,7 +3655,16 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
> > set_vm_area_page_order(area, page_shift - PAGE_SHIFT);
> > page_order = vm_area_page_order(area);
> >
> > - area->nr_pages = vm_area_alloc_pages(gfp_mask | __GFP_NOWARN,
> > + /*
> > + * Higher order nofail allocations are really expensive and
> > + * potentially dangerous (pre-mature OOM, disruptive reclaim
> > + * and compaction etc.
> > + *
> > + * Please note, the __vmalloc_node_range_noprof() falls-back
> > + * to order-0 pages if high-order attempt has been unsuccessful.
> > + */
> > + area->nr_pages = vm_area_alloc_pages(page_order ?
> > + gfp_mask &= ~__GFP_NOFAIL : gfp_mask | __GFP_NOWARN,
> > node, page_order, nr_small_pages, area->pages);
> >
> > atomic_long_add(area->nr_pages, &nr_vmalloc_pages);
> > <snip>
> >
> > Is that aligned with your wish?
>
> I am not a great fan of modifying gfp_mask inside the ternary operator
> like that. It makes the code harder to read. Is there any actual reason
> to simply drop GFP_NOFAIL unconditionally and rely do the NOFAIL
> handling for all orders at the same place?
>
1. So, for bulk we have below:

/* gfp_t bulk_gfp = gfp & ~__GFP_NOFAIL; */

I am not sure if we need it but it says it does not support it which
is not clear for me why we have to drop __GFP_NOFAIL for bulk(). There
is a fallback to a single page allocator. If passing __GFP_NOFAIL does
not trigger any warning or panic a system, then i do not follow why
we drop that flag.

Is that odd?

2. High-order allocations. Do you think we should not care much about
it when __GFP_NOFAIL is set? Same here, there is a fallback for order-0
if "high" fails, it is more likely NO_FAIL succeed for order-0. Thus
keeping NOFAIL for high-order sounds like not a good approach to me.

3. "... at the same place?"
Do you mean in the __vmalloc_node_range_noprof()?

__vmalloc_node_range_noprof()
-> __vmalloc_area_node(gfp_mask)
-> vm_area_alloc_pages()

if, so it is not straight forward, i.e. there is one more allocation:

<snip>
static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
pgprot_t prot, unsigned int page_shift,
int node)
{
...
/* Please note that the recursion is strictly bounded. */
if (array_size > PAGE_SIZE) {
area->pages = __vmalloc_node_noprof(array_size, 1, nested_gfp, node,
area->caller);
} else {
area->pages = kmalloc_node_noprof(array_size, nested_gfp, node);
}
...
}
<snip>

whereas it is easier to do it inside of the __vmalloc_area_node().

>
> Not that I care about this much TBH. It is an improvement to drop all
> the NOFAIL specifics from vm_area_alloc_pages.
>
I agree. I also do not like modifying gfp flags on different levels and
different cases. To me there is only one case. It is high-order requests
with NOFAIL. For this i think we should keep our approach, i mean
dropping NOFAIL and repeat because we have a fallback.

--
Uladzislau Rezki