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

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Message-id: <163524528594.8576.8070122002785265336@noble.neil.brown.name>
Date:   Tue, 26 Oct 2021 21:48:05 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Michal Hocko" <mhocko@...e.com>
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>
Subject: Re: [PATCH 4/4] mm: allow !GFP_KERNEL allocations for kvmalloc

On Tue, 26 Oct 2021, Michal Hocko wrote:
> On Tue 26-10-21 10:34:34, Neil Brown wrote:
> > 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
> 
> I thought naming the higher level gfp mask would be more helpful here.
> Most people do not tend to think in terms of __GFP_DIRECT_RECLAIM but
> rather GFP_NOWAIT or GFP_ATOMIC.

Maybe it would.  But the text says "Reclaim modifiers" and then lists
one modifier and one mask.  That is confusing.
If you want to mention both, keep them separate.

  GFP_NOWAIT and GFP_ATOMIC are not supported, neither is the
  __GFP_NORETRY modifier.

or something like that.

Thanks,
NeilBrown


> 
> > But I really cannot see why either of these statements are true.
> 
> The reason is same as why vmalloc do not support neither of them.
> 
> > 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.
> 
> __GFP_NORETRY is used internaly by kvmalloc but that doesn't mean it is
> supported by the caller. In fact __GFP_NORETRY is used to implement a
> higher level logic of the prioritization between kmalloc and vmalloc
> fallback because some users would rather see vmalloc fallback even for
> smaller allocations which do not really fail otherwise (e.g. < order-4).
> -- 
> Michal Hocko
> SUSE Labs
> 
>