linux-kernel - Re: [PATCH v2 2/2] mm: adds NOSIGBUS extension to mmap()

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Date:   Fri, 4 Jun 2021 18:24:07 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Ming Lin <mlin@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Simon Ser <contact@...rsion.fr>,
        Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org
Subject: Re: [PATCH v2 2/2] mm: adds NOSIGBUS extension to mmap()

On Fri, Jun 04, 2021 at 12:43:22AM -0700, Ming Lin wrote:
> Adds new flag MAP_NOSIGBUS of mmap() to specify the behavior of
> "don't SIGBUS on fault". Right now, this flag is only allowed
> for private mapping.

That's not what your use case asks for.

SIGBUS can be generated for a number of reasons, not only on fault beyond
end-of-file. vmf_error() would convert any errno, except ENOMEM to
VM_FAULT_SIGBUS.

Do you want to ignore -EIO or -ENOSPC? I don't think so.

> For MAP_NOSIGBUS mapping, map in the zero page on read fault
> or fill a freshly allocated page with zeroes on write fault.

I don't like the resulting semantics: if you had a read fault beyond EOF
and got zero page, you will still see zero page even if the file grows.
Yes, it's allowed by POSIX for MAP_PRIVATE to get out-of-sync with the
file, but it's not what users used to.

It might be enough for the use case, but I would rather avoid one-user
features.

-- 
 Kirill A. Shutemov