linux-kernel - Re: [PATCH v2] memcg: allow exiting tasks to write back data to swap

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Message-ID: <193d98b0d5d2b14da1b96953fcb5d91b2a35bf21.camel@surriel.com>
Date: Tue, 14 Jan 2025 12:11:54 -0500
From: Rik van Riel <riel@...riel.com>
To: Michal Hocko <mhocko@...e.com>
Cc: Johannes Weiner <hannes@...xchg.org>, Yosry Ahmed
 <yosryahmed@...gle.com>,  Balbir Singh <balbirs@...dia.com>, Roman Gushchin
 <roman.gushchin@...ux.dev>, hakeel Butt	 <shakeel.butt@...ux.dev>, Muchun
 Song <muchun.song@...ux.dev>, Andrew Morton	 <akpm@...ux-foundation.org>,
 cgroups@...r.kernel.org, linux-mm@...ck.org, 	linux-kernel@...r.kernel.org,
 kernel-team@...a.com, Nhat Pham <nphamcs@...il.com>
Subject: Re: [PATCH v2] memcg: allow exiting tasks to write back data to swap

On Tue, 2025-01-14 at 18:00 +0100, Michal Hocko wrote:
> On Tue 14-01-25 11:51:18, Rik van Riel wrote:
> > On Tue, 2025-01-14 at 17:46 +0100, Michal Hocko wrote:
> > > On Tue 14-01-25 11:09:55, Johannes Weiner wrote:
> > > 
> > > > charge_memcg
> > > > mem_cgroup_swapin_charge_folio
> > > > __read_swap_cache_async
> > > > swapin_readahead
> > > > do_swap_page
> > > > handle_mm_fault
> > > > do_user_addr_fault
> > > > exc_page_fault
> > > > asm_exc_page_fault
> > > > __get_user
> > > 
> > > All the way here and return the failure to futex_cleanup which
> > > doesn't
> > > retry __get_user on the failure AFAICS (exit_robust_list). But I
> > > might
> > > be missing something, it's been quite some time since I've looked
> > > into
> > > futex code.
> > 
> > Can you explain how -ENOMEM would get propagated down
> > past the page fault handler?
> > 
> > This isn't get_user_pages(), which can just pass
> > -ENOMEM on to the caller.
> > 
> > If there is code to pass -ENOMEM on past the page
> > fault exception handler, I have not been able to
> > find it. How does this work?
> 
> This might be me misunderstading get_user machinery but doesn't it
> return a failure on PF handler returing ENOMEM?

I believe __get_user simply does a memcpy, and ends
up in the page fault handler.

It does not access userspace explicitly like we do
with functions like get_user_pages.

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