linux-kernel - Re: [PATCH 4/4] mm: allow !GFP_KERNEL allocations for kvmalloc

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Message-id: <163520487423.16092.18303917539436351482@noble.neil.brown.name>
Date:   Tue, 26 Oct 2021 10:34:34 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Michal Hocko" <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, "Dave Chinner" <david@...morbit.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Christoph Hellwig" <hch@...radead.org>,
        "Uladzislau Rezki" <urezki@...il.com>,
        linux-fsdevel@...r.kernel.org,
        "LKML" <linux-kernel@...r.kernel.org>,
        "Ilya Dryomov" <idryomov@...il.com>,
        "Jeff Layton" <jlayton@...nel.org>,
        "Michal Hocko" <mhocko@...e.com>
Subject: Re: [PATCH 4/4] mm: allow !GFP_KERNEL allocations for kvmalloc

On Tue, 26 Oct 2021, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> A support for GFP_NO{FS,IO} and __GFP_NOFAIL has been implemented
> by previous patches so we can allow the support for kvmalloc. This
> will allow some external users to simplify or completely remove
> their helpers.
> 
> GFP_NOWAIT semantic hasn't been supported so far but it hasn't been
> explicitly documented so let's add a note about that.
> 
> ceph_kvmalloc is the first helper to be dropped and changed to
> kvmalloc.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.com>
> ---
>  include/linux/ceph/libceph.h |  1 -
>  mm/util.c                    | 15 ++++-----------
>  net/ceph/buffer.c            |  4 ++--
>  net/ceph/ceph_common.c       | 27 ---------------------------
>  net/ceph/crypto.c            |  2 +-
>  net/ceph/messenger.c         |  2 +-
>  net/ceph/messenger_v2.c      |  2 +-
>  net/ceph/osdmap.c            | 12 ++++++------
>  8 files changed, 15 insertions(+), 50 deletions(-)
> 
> diff --git a/include/linux/ceph/libceph.h b/include/linux/ceph/libceph.h
> index 409d8c29bc4f..309acbcb5a8a 100644
> --- a/include/linux/ceph/libceph.h
> +++ b/include/linux/ceph/libceph.h
> @@ -295,7 +295,6 @@ extern bool libceph_compatible(void *data);
>  
>  extern const char *ceph_msg_type_name(int type);
>  extern int ceph_check_fsid(struct ceph_client *client, struct ceph_fsid *fsid);
> -extern void *ceph_kvmalloc(size_t size, gfp_t flags);
>  
>  struct fs_parameter;
>  struct fc_log;
> diff --git a/mm/util.c b/mm/util.c
> index bacabe446906..fdec6b4b1267 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -549,13 +549,10 @@ EXPORT_SYMBOL(vm_mmap);
>   * Uses kmalloc to get the memory but if the allocation fails then falls back
>   * to the vmalloc allocator. Use kvfree for freeing the memory.
>   *
> - * Reclaim modifiers - __GFP_NORETRY and __GFP_NOFAIL are not supported.
> + * Reclaim modifiers - __GFP_NORETRY and GFP_NOWAIT are not supported.

GFP_NOWAIT is not a modifier.  It is a base value that can be modified.
I think you mean that
    __GFP_NORETRY is not supported and __GFP_DIRECT_RECLAIM is required

But I really cannot see why either of these statements are true.

Before your patch, __GFP_NORETRY would have forced use of kmalloc, so
that would mean it isn't really supported.  But that doesn't happen any more.

Thanks,
NeilBrown