linux-kernel - Re: question about mmap MAP_PRIVATE on PMEM/DAX/fs files

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Message-ID: <CAPcyv4jhkkUd_rZ9PhZhOZDYYgMj28siVBUjhMMT4c4HJiaAHw@mail.gmail.com>
Date:   Wed, 6 Feb 2019 15:42:17 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Larry Bassel <larry.bassel@...cle.com>
Cc:     linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: question about mmap MAP_PRIVATE on PMEM/DAX/fs files

On Wed, Feb 6, 2019 at 3:17 PM Larry Bassel <larry.bassel@...cle.com> wrote:
>
> Is mmaping a PMEM/DAX/fs file MAP_PRIVATE supported?

Yes.

> Is it something
> that people are likely to want to do?

MAP_PRIVATE for file backed mappings is useful for processes to read
common data as input, but hide their writes to that data.

> If it is supported, suppose I open a file in PMEM/DAX/fs, mmap it
> MAP_PRIVATE, read from the memory mapped file (with memory accesses,
> not the read syscall) and take a page fault which the kernel satisfies.
>
> At this time do my page tables for the private mmaped page(s) point to the
> PMEM corresponding to the file and the kernel will wait until
> the page(s) is/are altered (either by me or someone else) to
> copy on write and give me a different page/mapping?

For this example let's assume the filesystem already allocated
capacity for the file at the given offset, otherwise the kernel is
free to map in a zero page to cover holes in the file.

> Or does the kernel avoid this by always mapping a copy of the
> page(s) involved in the private mmap in the first place?
>

At read-fault time the page tables point to the direct PMEM page for
the file allocation, for a write fault a "System RAM" page is mapped
in place of PMEM.

> In either case, is my private copy going to come from PMEM or is it
> an "ordinary" page, or is this "random"? Does the program have
> any choice in this (i.e. suppose I want to make sure my copied
> page is persistent)?

No, the copied page will never be persisted. From the mmap() man page:

"Updates to the mapping are not visible to other processes mapping the
same file, and are not carried through to the underlying file."