linux-kernel - Re: [PATCH v2 bpf 1/3] vmalloc: replace VM_NO_HUGE_VMAP with VM_ALLOW_HUGE

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Date:   Mon, 11 Apr 2022 23:00:53 -0700
From:   Song Liu <song@...nel.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     bpf <bpf@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
        open list <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        rick.p.edgecombe@...el.com, imbrenda@...ux.ibm.com,
        Luis Chamberlain <mcgrof@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH v2 bpf 1/3] vmalloc: replace VM_NO_HUGE_VMAP with VM_ALLOW_HUGE_VMAP

On Mon, Apr 11, 2022 at 9:18 PM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Mon, Apr 11, 2022 at 04:35:46PM -0700, Song Liu wrote:
> > Huge page backed vmalloc memory could benefit performance in many cases.
> > Since some users of vmalloc may not be ready to handle huge pages,
> > VM_NO_HUGE_VMAP was introduced to allow vmalloc users to opt-out huge
> > pages. However, it is not easy to add VM_NO_HUGE_VMAP to all the users
> > that may try to allocate >= PMD_SIZE pages, but are not ready to handle
> > huge pages properly.
>
> This is a good place to document what the problems are, and how they are
> hard to track down (e.g. because the allocations are passed down I/O
> stacks)

Will add it in v3.

>
> >
> > Replace VM_NO_HUGE_VMAP with an opt-in flag, VM_ALLOW_HUGE_VMAP, so that
> > users that benefit from huge pages could ask specificially.
> >
> > Also, replace vmalloc_no_huge() with opt-in helper vmalloc_huge().
>
> We still need to find out what the primary users of the large vmalloc
> hashes was and convert them.

@ Claudio and Nicholas,

Could you please help identify users of large vmalloc? So far, I found
alloc_large_system_hash(), and something like the following seems to
work:

diff --git i/mm/page_alloc.c w/mm/page_alloc.c
index 6e5b4488a0c5..20d38b8482c4 100644
--- i/mm/page_alloc.c
+++ w/mm/page_alloc.c
@@ -8919,7 +8919,7 @@ void *__init alloc_large_system_hash(const char
*tablename,
                                table = memblock_alloc_raw(size,
                                                           SMP_CACHE_BYTES);
                } else if (get_order(size) >= MAX_ORDER || hashdist) {
-                       table = __vmalloc(size, gfp_flags);
+                       table = __vmalloc_huge(size, gfp_flags);
                        virt = true;
                        if (table)
                                huge = is_vm_area_hugepages(table);
diff --git i/mm/vmalloc.c w/mm/vmalloc.c
index 7cc2be6a7554..cbadbe83e6a6 100644
--- i/mm/vmalloc.c
+++ w/mm/vmalloc.c
@@ -3253,6 +3253,14 @@ void *__vmalloc(unsigned long size, gfp_t gfp_mask)
 }
 EXPORT_SYMBOL(__vmalloc);

+void *__vmalloc_huge(unsigned long size, gfp_t gfp_mask)
+{
+       return __vmalloc_node_range(size, 1, VMALLOC_START, VMALLOC_END,
+                                   gfp_mask, PAGE_KERNEL, VM_ALLOW_HUGE_VMAP,
+                                   NUMA_NO_NODE, __builtin_return_address(0));
+}
+EXPORT_SYMBOL_GPL(__vmalloc_huge);
+
 /**
  * vmalloc - allocate virtually contiguous memory
  * @size:    allocation size


>
> > +extern void *vmalloc_huge(unsigned long size) __alloc_size(1);
>
> No need for the extern.
>
> > +EXPORT_SYMBOL(vmalloc_huge);
>
> EXPORT_SYMBOL_GPL for all advanced vmalloc functionality, please.

Will fix these in v3.

Thanks,
Song