Re: [PATCH RFC] alloc_tag: add option to pick the first codetag along callchain

["David Wang" <00107082@163.com>]

At 2026-01-07 07:26:18, "Kent Overstreet" <kent.overstreet@linux.dev> wrote:
>On Tue, Jan 06, 2026 at 10:07:36PM +0800, David Wang wrote:
>> I agree, the accounting would be incorrect for alloc sites down the callchain, and would confuse things.
>> When the call chain has more than one codetag, correct accounting for one codetag would always mean incorrect
>> accounting for other codetags, right? But I don't think picking the first tag would make the accounting totally incorrect. 
>
>The trouble is you end up in situations where you have an alloc tag on
>the stack, but then you're doing an internal allocation that definitely
>should not be accounted to the outer alloc tag.
>
>E.g. there's a lot of internal mm allocations like this; object
>extension vectors was I think the first place where it came up,
>vmalloc() also has its own internal data structures that require
>allocations.
>
>Just using the outermost tag means these inner allocations will get
>accounted to other unrelated alloc tags _effectively at random_; meaning
>if we're burning more memory than we should be in a place like that it
>will never show up in a way that we'll notice and be able to track it
>down.

Kind of feel that the same thing could be said for drivers: the driver could use more memory
than the data says....this is actually true....
Different developer may have different focus concerning the allocation site.

>
>> Totally agree.
>> I used to sum by filepath prefix to aggregate memory usage for drivers.
>> Take usb subsystem for example,  on my system, the data say my usb drivers use up 200k memory,
>> and if pick first codetag, the data say ~350K.   Which one is lying, or are those two both lying. I am  confused.
>> 
>> I think this also raises the question of what is the *correct* way to make use of /proc/allocinfo...
>
>So yes, summing by filepath prefix is the way we want things to work.
>
>But getting there - with a fully reliable end result - is a process.
>
>What you want to do is - preferably on a reasonably idle machine, aside
>from the code you're looking at - just look at everything in
>/proc/allocinfo and sort by size. Look at the biggest ones that might be
>relevant to your subsystem, and look for any that are suspicious and
>perhaps should be accounted to your code. Yes, that may entail reading
>code :)
>
>This is why accounting to the innermost tag is important - by doing it
>this way, if an allocation is being accounted at the wrong callsite
>they'll all be lumped together at the specific callsite that needs to be
>fixed, which then shows up higher than normal in /proc/allocations, so
>that it gets looked at.
>
>> >The fact that you have to be explicit about where the accounting happens
>> >via _noprof is a feature, not a bug :)
>> 
>> But it is tedious... :(
>
>That's another way of saying it's easy :)
>
>Spot an allocation with insufficiently fine grained accounting and it's
>generally a 3-5 line patch to fix it, I've been doing those here and
>there - e.g. mempools, workqueues, rhashtables.
>
>One trick I did with rhashtables that may be relevant to other
>subsystems: rhashtable does background processing for your hash table,
>which will do new allocations for your hash table out of a workqueue.
>
>So rhashtable_init() gets wrapped in alloc_hooks(), and then it stashes
>the pointer to that alloc tag in the rhashtable, and uses it later for
>all those asynchronous allocations.
>
>This means that instead of seeing a ton of memory accounted to the
>rhashtable code, with no idea of which rhashtable is burning memory -
>all the rhashtable allocations are accounted to the callsit of the
>initialization, meaning it's trivial to see which one is burning memory.

Not that easy, ....code keeps being refactored, _noprof need to be changed along.
I was trying to split the accounting for __filemap_get_folio to its callers in 6.18,  
it was easy,  only ~10 lines of code changes. But 6.19 starts with code refactors to
__filemap_get_folio, adding another level of indirection, allocation callchain becomes
longer, and more _noprof should be added...quite unpleasant...

Sometimes I would feel too many _noprof could be obstacle for future code refactors....

PS: There are several allocation sites have *huge* memory accounting, __filemap_get_folio is
one of those. splitting those accounting to its callers would be more informative


Thanks
David