On Mon 26-08-24 14:38:40, Uladzislau Rezki wrote:
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,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?
- 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?
I suspect this was a pre-caution more than anything.
- 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.
We should avoid high order allocations with GFP_NOFAIL at all cost.
- "... 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().
Right. The allocation path is quite convoluted here. If it is just too much of a hassle to implement NOFAIL at a single place then we should aim at reducing that. Having that at 3 different layers is just begging for inconsistences.