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Date: Fri, 16 Feb 2024 19:49:01 +0100
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Subject: Re: [PATCH v3 07/35] mm/slab: introduce SLAB_NO_OBJ_EXT to avoid
 obj_ext creation
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To: Suren Baghdasaryan <surenb@google.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>, akpm@linux-foundation.org,
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From: Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <CAJuCfpGBCNsvK35Bq8666cJeZ3Hwfwj6mDJ6M5Wjg7oZi8xd0g@mail.gmail.com>
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On 2/16/24 19:41, Suren Baghdasaryan wrote:
> On Thu, Feb 15, 2024 at 10:10 PM Suren Baghdasaryan <surenb@google.com> wrote:
>>
>> On Thu, Feb 15, 2024 at 1:50 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>> >
>> > On 2/15/24 22:37, Kent Overstreet wrote:
>> > > On Thu, Feb 15, 2024 at 10:31:06PM +0100, Vlastimil Babka wrote:
>> > >> On 2/12/24 22:38, Suren Baghdasaryan wrote:
>> > >> > Slab extension objects can't be allocated before slab infrastructure is
>> > >> > initialized. Some caches, like kmem_cache and kmem_cache_node, are created
>> > >> > before slab infrastructure is initialized. Objects from these caches can't
>> > >> > have extension objects. Introduce SLAB_NO_OBJ_EXT slab flag to mark these
>> > >> > caches and avoid creating extensions for objects allocated from these
>> > >> > slabs.
>> > >> >
>> > >> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>> > >> > ---
>> > >> >  include/linux/slab.h | 7 +++++++
>> > >> >  mm/slub.c            | 5 +++--
>> > >> >  2 files changed, 10 insertions(+), 2 deletions(-)
>> > >> >
>> > >> > diff --git a/include/linux/slab.h b/include/linux/slab.h
>> > >> > index b5f5ee8308d0..3ac2fc830f0f 100644
>> > >> > --- a/include/linux/slab.h
>> > >> > +++ b/include/linux/slab.h
>> > >> > @@ -164,6 +164,13 @@
>> > >> >  #endif
>> > >> >  #define SLAB_TEMPORARY            SLAB_RECLAIM_ACCOUNT    /* Objects are short-lived */
>> > >> >
>> > >> > +#ifdef CONFIG_SLAB_OBJ_EXT
>> > >> > +/* Slab created using create_boot_cache */
>> > >> > +#define SLAB_NO_OBJ_EXT         ((slab_flags_t __force)0x20000000U)
>> > >>
>> > >> There's
>> > >>    #define SLAB_SKIP_KFENCE        ((slab_flags_t __force)0x20000000U)
>> > >> already, so need some other one?
>>
>> Indeed. I somehow missed it. Thanks for noticing, will fix this in the
>> next version.
> 
> Apparently the only unused slab flag is 0x00000200U, all others seem
> to be taken. I'll use it if there are no objections.

OK. Will look into the cleanup and consolidation - we already know
SLAB_MEM_SPREAD became dead with SLAB removed. If it comes to worst, we can
switch to 64 bits again.

>>
>> > >
>> > > What's up with the order of flags in that file? They don't seem to
>> > > follow any particular ordering.
>> >
>> > Seems mostly in increasing order, except commit 4fd0b46e89879 broke it for
>> > SLAB_RECLAIM_ACCOUNT?
>> >
>> > > Seems like some cleanup is in order, but any history/context we should
>> > > know first?
>> >
>> > Yeah noted, but no need to sidetrack you.