linux-kernel - Re: [PATCH] mm: append __GFP_COMP flag for trace

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Date:   Tue, 27 Apr 2021 11:29:32 +0800
From:   Xiongwei Song <sxwjean@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Xiongwei Song <sxwjean@...com>, cl@...ux.com, penberg@...nel.org,
        rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, vbabka@...e.cz, linux-mm@...ck.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: append __GFP_COMP flag for trace_malloc

On Tue, Apr 27, 2021 at 10:54 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Tue, Apr 27, 2021 at 10:43:20AM +0800, Xiongwei Song wrote:
> > From: Xiongwei Song <sxwjean@...il.com>
> >
> > When calling kmalloc_order, the flags should include __GFP_COMP here,
> > so that trace_malloc can trace the precise flags.
>
> I suppose that depends on your point of view.
Correct.

Should we report the
> flags used by the caller, or the flags that we used to allocate memory?
> And why does it matter?
When I capture kmem:kmalloc events on my env with perf:
(perf record -p my_pid -e kmem:kmalloc)
I got the result below:
     0.08%  call_site=ffffffff851d0cb0 ptr=0xffff8c04a4ca0000
bytes_req=10176 bytes_alloc=16384
gfp_flags=GFP_ATOMIC|__GFP_NOWARN|__GFP_NOMEMALLOC
     0.08%  call_site=ffffffff851d0cb0 ptr=0xffff8c04a4ca4000
bytes_req=10176 bytes_alloc=16384
gfp_flags=GFP_ATOMIC|__GFP_NOWARN|__GFP_NOMEMALLOC
     0.08%  call_site=ffffffff851d0cb0 ptr=0xffff8c04a4ca8000
bytes_req=10176 bytes_alloc=16384
gfp_flags=GFP_ATOMIC|__GFP_NOWARN|__GFP_NOMEMALLOC
     0.08%  call_site=ffffffff851d0cb0 ptr=0xffff8c04a6f80000
bytes_req=10176 bytes_alloc=16384
gfp_flags=GFP_ATOMIC|__GFP_NOWARN|__GFP_NOMEMALLOC
     0.08%  call_site=ffffffff851d0cb0 ptr=0xffff8c04a6f84000
bytes_req=10176 bytes_alloc=16384
gfp_flags=GFP_ATOMIC|__GFP_NOWARN|__GFP_NOMEMALLOC
     0.08%  call_site=ffffffff851d0cb0 ptr=0xffff8c04a6f88000
bytes_req=10176 bytes_alloc=16384
gfp_flags=GFP_ATOMIC|__GFP_NOWARN|__GFP_NOMEMALLOC
     0.08%  call_site=ffffffff851d0cb0 ptr=0xffff8c04a6f8c000
bytes_req=10176 bytes_alloc=16384
gfp_flags=GFP_ATOMIC|__GFP_NOWARN|__GFP_NOMEMALLOC
     0.07%  call_site=ffffffff851d0cb0 ptr=0xffff8c04a4c80000
bytes_req=10176 bytes_alloc=16384
gfp_flags=GFP_ATOMIC|__GFP_NOWARN|__GFP_NOMEMALLOC

The value of gfp_flags made me confused, I spent some time to find out
which trace_malloc
is here. So I think we should append __GFP_COMP.

Regards