linux-kernel - Re: [PATCH 1/4] mm: Trial do_wp

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Date:   Fri, 18 Sep 2020 13:59:41 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, John Hubbard <jhubbard@...dia.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Maya B . Gokhale" <gokhale2@...l.gov>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Marty Mcfadden <mcfadden8@...l.gov>,
        Kirill Shutemov <kirill@...temov.name>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Jan Kara <jack@...e.cz>, Kirill Tkhai <ktkhai@...tuozzo.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

On Fri, Sep 18, 2020 at 1:41 PM Peter Xu <peterx@...hat.com> wrote:
>
> What would be the result if we simply use GFP_ATOMIC?  Would there be too many
> pages to allocate in bulk for ATOMIC?

It's very easy to run out of memory with GFP_ATOMIC, and also cause
various nasty issues with networking (ie when you've depleted atomic
memory, networking starts losing packets etc).

So yeah, this code should not use GFP_ATOMIC, I think it just needs to
drop and re-take the paeg table lock.

Which is easy enough to do: returning a zero 'entry.val' already does
that for other reasons, there's nothing magical about holding the lock
here, there's no larger page table lock requirement.

The main annoyance is that I think it means that copy_pte_range()
would also have to have a special "preallocation page" thing for this
case, so that it can drop the lock, do the allocation, and then take
the lock again and return 0 (to repeat - now with the preallocation
filled).

Honestly, if we had a completely *reliable* sign of "this page is
pinned", then I think the much nicer option would be to just say
"pinned pages will not be copied at all". Kind of an implicit
VM_DONTCOPY.

(Or we'd do the reverse, and say "pinned pages stay pinned even in the child").

But that's not an option when the pinning test is a "maybe". Oh well.

             Linus