linux-kernel - Re: [PATCH 35/40] lib: implement context capture support for tagged allocations

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Message-ID: <CAJuCfpGUtw6cbjLsksGJKATZfTV0FEYRXwXT0pZV83XqQydBgg@mail.gmail.com>
Date:   Thu, 4 May 2023 09:22:07 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
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Subject: Re: [PATCH 35/40] lib: implement context capture support for tagged allocations

On Thu, May 4, 2023 at 1:09 AM Michal Hocko <mhocko@...e.com> wrote:
>
> On Wed 03-05-23 08:24:19, Suren Baghdasaryan wrote:
> > On Wed, May 3, 2023 at 12:39 AM Michal Hocko <mhocko@...e.com> wrote:
> > >
> > > On Mon 01-05-23 09:54:45, Suren Baghdasaryan wrote:
> > > [...]
> > > > +struct codetag_ctx *alloc_tag_create_ctx(struct alloc_tag *tag, size_t size)
> > > > +{
> > > > +     struct alloc_call_ctx *ac_ctx;
> > > > +
> > > > +     /* TODO: use a dedicated kmem_cache */
> > > > +     ac_ctx = kmalloc(sizeof(struct alloc_call_ctx), GFP_KERNEL);
> > >
> > > You cannot really use GFP_KERNEL here. This is post_alloc_hook path and
> > > that has its own gfp context.
> >
> > I missed that. Would it be appropriate to use the gfp_flags parameter
> > of post_alloc_hook() here?
>
> No. the original allocation could have been GFP_USER based and you do
> not want these allocations to pullute other zones potentially. You want
> GFP_KERNEL compatible subset of that mask.

Ack.

>
> But even then I really detest an additional allocation from this context
> for every single allocation request. There GFP_NOWAIT allocation for
> steckdepot but that is at least cached and generally not allocating.
> This will allocate for every single allocation.

A small correction here. alloc_tag_create_ctx() is used only for
allocations which we requested to capture the context. So, this last
sentence is true for allocations we specifically marked to capture the
context, not in general.

> There must be a better way.

Yeah, agree, it would be good to avoid allocations in this path. Any
specific ideas on how to improve this? Pooling/caching perhaps? I
think kmem_cache does some of that already but maybe something else?
Thanks,
Suren.

> --
> Michal Hocko
> SUSE Labs