linux-kernel - Re: [PATCH 2/8] mm: Split huge pages on write-notify or COW

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Date:   Sat, 29 Feb 2020 20:04:42 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, pv-drivers@...are.com,
        linux-graphics-maintainer@...are.com,
        Thomas Hellstrom <thellstrom@...are.com>,
        Michal Hocko <mhocko@...e.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Christian König <christian.koenig@....com>
Subject: Re: [PATCH 2/8] mm: Split huge pages on write-notify or COW

On Tue,  3 Dec 2019 14:22:33 +0100 Thomas Hellström (VMware) <thomas_os@...pmail.org> wrote:

> We currently only do COW and write-notify on the PTE level, so if the
> huge_fault() handler returns VM_FAULT_FALLBACK on wp faults,
> split the huge pages and page-table entries. Also do this for huge PUDs
> if there is no huge_fault() handler and the vma is not anonymous, similar
> to how it's done for PMDs.

There is no description here of *why* we are making this change?

> Note that fs/dax.c does the splitting in the huge_fault() handler, but as
> huge_fault() is implemented by modules we need to consider whether to
> export the splitting functions for use in the modules or whether to try
> to keep calls in the core. Opt for the latter. A follow-up patch can
> remove the dax.c split_huge_pmd() if needed.