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

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Date:   Fri, 7 May 2021 13:41:44 +0800
From:   Xiongwei Song <sxwjean@...il.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Xiongwei Song <sxwjean@...com>, cl@...ux.com,
        penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, 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 Mon, May 3, 2021 at 8:35 PM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 4/28/21 5:05 AM, Xiongwei Song wrote:
> > On Tue, Apr 27, 2021 at 7:26 PM Matthew Wilcox <willy@...radead.org> wrote:
> >>
> >> On Tue, Apr 27, 2021 at 01:30:48PM +0800, Xiongwei Song wrote:
> >> > Hi Mattew,
> >> >
> >> > One more thing I should explain, the kmalloc_order() appends the
> >> > __GFP_COMP flags,
> >> > not by the caller.
> >> >
> >> > void *kmalloc_order(size_t size, gfp_t flags, unsigned int order)
> >> > {
> >> > ...........................................................
> >> >
> >> > flags |= __GFP_COMP;
> >> > page = alloc_pages(flags, order);
> >> > ...........................................................
> >> > return ret;
> >> > }
> >> > EXPORT_SYMBOL(kmalloc_order);
> >> >
> >> > #ifdef CONFIG_TRACING
> >> > void *kmalloc_order_trace(size_t size, gfp_t flags, unsigned int order)
> >> > {
> >> > void *ret = kmalloc_order(size, flags, order);
> >> > trace_kmalloc(_RET_IP_, ret, size, PAGE_SIZE << order, flags);
> >> > return ret;
> >> > }
> >> > EXPORT_SYMBOL(kmalloc_order_trace);
> >> > #endif
> >>
> >> Yes, I understood that.  What I don't understand is why appending the
> >> __GFP_COMP to the trace would have been less confusing for you.
> >>
> >> Suppose I have some code which calls:
> >>
> >>         kmalloc(10 * 1024, GFP_ATOMIC|__GFP_NOWARN|__GFP_NOMEMALLOC);
> >>
> >> and I see in my logs
> >>
> >>      0.08%  call_site=ffffffff851d0cb0 ptr=0xffff8c04a4ca0000 bytes_req=10176 bytes_alloc=16384 gfp_flags=GFP_ATOMIC|__GFP_NOWARN|__GFP_NOMEMALLOC|__GFP_COMP
> >>
> >> That seems to me _more_ confusing because I would wonder "Where did that
> >> __GFP_COMP come from?"
> >
> > Thank you for the comments. But I disagree.
>
> FTR, I agree with Matthew. This is a tracepoint for kmalloc() so I would expect
> to see what flags were passed to kmalloc().
> If I wanted to see how the flags translated to page allocator's flags, I would
> have used a page allocator's tracepoint which would show me that.

Make sense. Thank you.

> > When I use trace, I hope I can get the precise data rather than something
> > changed that I don't know , then I can get the correct conclusion or
> > direction on my issue.
>
> It's precise from the point of the caller.
>
> > Here my question is what the trace events are for if they don't provide the
> > real situation? I think that's not graceful and friendly.
> >
> > From my perspective, it'd be better to know my flags changed before checking
> > code lines one by one. In other words, I need a warning to reminder me on this,
> > then I can know quickly my process might do some incorrect things.
>
> Your process should not care about __GFP_COMP if you use properly
> kmalloc()+kfree(). Once you start caring about __GFP_COMP, you should be using
> page allocator's API, not kmalloc().
>
> > Regards,
> > Xiongwei
> >
>