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From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Tue, 26 Oct 2021 17:48:32 +0200
Message-ID: <CA+KHdyVqOuKny7bT+CtrCk8BrnARYz744Ze6cKMuy2BXo5e7jw@mail.gmail.com>
Subject: Re: [PATCH 2/4] mm/vmalloc: add support for __GFP_NOFAIL
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Linux Memory Management List <linux-mm@kvack.org>,
        Dave Chinner <david@fromorbit.com>, Neil Brown <neilb@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christoph Hellwig <hch@infradead.org>,
        linux-fsdevel@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Ilya Dryomov <idryomov@gmail.com>,
        Jeff Layton <jlayton@kernel.org>,
        Michal Hocko <mhocko@suse.com>
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> From: Michal Hocko <mhocko@suse.com>
>
> Dave Chinner has mentioned that some of the xfs code would benefit from
> kvmalloc support for __GFP_NOFAIL because they have allocations that
> cannot fail and they do not fit into a single page.
>
> The larg part of the vmalloc implementation already complies with the
> given gfp flags so there is no work for those to be done. The area
> and page table allocations are an exception to that. Implement a retry
> loop for those.
>
> Add a short sleep before retrying. 1 jiffy is a completely random
> timeout. Ideally the retry would wait for an explicit event - e.g.
> a change to the vmalloc space change if the failure was caused by
> the space fragmentation or depletion. But there are multiple different
> reasons to retry and this could become much more complex. Keep the retry
> simple for now and just sleep to prevent from hogging CPUs.
>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  mm/vmalloc.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index c6cc77d2f366..602649919a9d 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2941,8 +2941,12 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
>         else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == 0)
>                 flags = memalloc_noio_save();
>
> -       ret = vmap_pages_range(addr, addr + size, prot, area->pages,
> +       do {
> +               ret = vmap_pages_range(addr, addr + size, prot, area->pages,
>                         page_shift);
> +               if (ret < 0)
> +                       schedule_timeout_uninterruptible(1);
> +       } while ((gfp_mask & __GFP_NOFAIL) && (ret < 0));
>

1.
After that change a below code:

<snip>
if (ret < 0) {
    warn_alloc(orig_gfp_mask, NULL,
        "vmalloc error: size %lu, failed to map pages",
        area->nr_pages * PAGE_SIZE);
    goto fail;
}
<snip>

does not make any sense anymore.

2.
Can we combine two places where we handle __GFP_NOFAIL into one place?
That would look like as more sorted out.

-- 
Uladzislau Rezki