From: Daniel Colascione <dancol@google.com> To: linux-mm@kvack.org Cc: Tim Murray <timmurray@google.com>, Minchan Kim <minchan@kernel.org> Subject: Why do we let munmap fail? Date: Mon, 21 May 2018 15:07:36 -0700 Message-ID: <CAKOZuetOD6MkGPVvYFLj5RXh200FaDyu3sQqZviVRhTFFS3fjA@mail.gmail.com> (raw) Right now, we have this system knob max_map_count that caps the number of VMAs we can have in a single address space. Put aside for the moment of whether this knob should exist: even if it does, enforcing it for munmap, mprotect, etc. produces weird and counter-intuitive situations in which it's possible to fail to return resources (address space and commit charge) to the system. At a deep philosophical level, that's the kind of operation that should never fail. A library that does all the right things can still experience a failure to deallocate resources it allocated itself if it gets unlucky with VMA merging. Why should we allow that to happen? Now let's return to max_map_count itself: what is it supposed to achieve? If we want to limit application kernel memory resource consumption, let's limit application kernel memory resource consumption, accounting for it on a byte basis the same way we account for other kernel objects allocated on behalf of userspace. Why should we have a separate cap just for the VMA count? I propose the following changes: 1) Let -1 mean "no VMA count limit". 2) Default max_map_count to -1. 3) Do not enforce max_map_count on munmap and mprotect. Alternatively, can we account VMAs toward max_map_count on a page count basis instead of a VMA basis? This way, no matter how you split and merge your VMAs, you'll never see a weird failure to release resources. We'd have to bump the default value of max_map_count to compensate for its new interpretation.
next reply index Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-05-21 22:07 Daniel Colascione [this message] 2018-05-21 22:12 ` Dave Hansen 2018-05-21 22:20 ` Daniel Colascione 2018-05-21 22:29 ` Dave Hansen 2018-05-21 22:35 ` Daniel Colascione 2018-05-21 22:48 ` Dave Hansen 2018-05-21 22:54 ` Daniel Colascione 2018-05-21 23:02 ` Dave Hansen 2018-05-21 23:16 ` Daniel Colascione 2018-05-21 23:32 ` Dave Hansen 2018-05-22 0:00 ` Daniel Colascione 2018-05-22 0:22 ` Matthew Wilcox 2018-05-22 0:38 ` Daniel Colascione 2018-05-22 1:19 ` Theodore Y. Ts'o 2018-05-22 1:41 ` Daniel Colascione 2018-05-22 2:09 ` Daniel Colascione 2018-05-22 2:11 ` Matthew Wilcox 2018-05-22 1:22 ` Matthew Wilcox 2018-05-22 5:34 ` Nicholas Piggin
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