linux-kernel - Re: [PATCH 1/3] mm/mincore: make mincore() more conservative

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Date:   Thu, 31 Jan 2019 10:51:21 +0100
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Michal Hocko <mhocko@...nel.org>, Josh Snyder <joshs@...flix.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-api@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Jann Horn <jannh@...gle.com>, Jiri Kosina <jkosina@...e.cz>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Chinner <david@...morbit.com>,
        Kevin Easton <kevin@...rana.org>,
        Matthew Wilcox <willy@...radead.org>,
        Cyril Hrubis <chrubis@...e.cz>, Tejun Heo <tj@...nel.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Daniel Gruss <daniel@...ss.cc>, Jiri Kosina <jikos@...nel.org>
Subject: Re: [PATCH 1/3] mm/mincore: make mincore() more conservative

Michal Hocko wrote on Thu, Jan 31, 2019:
> > Change the semantics of mincore() so that it only reveals pagecache information
> > for non-anonymous mappings that belog to files that the calling process could
> > (if it tried to) successfully open for writing.
> 
> I agree that this is a better way than the original 574823bfab82
> ("Change mincore() to count "mapped" pages rather than "cached" pages").
> One thing is still not clear to me though. Is the new owner/writeable
> check OK for the Netflix-like usecases? I mean does happycache have
> appropriate access to the cache data? I have tried to re-read the
> original thread but couldn't find any confirmation.

It's enough for my use cases and Josh didn't seem to oppose, but since
he's not in Cc I don't think he would answer -- added him now :)

FWIW happycache writes in the current directory by default so I assume
in the way they use it it would usually have access one way or another.

> If this still doesn't help happycache kind of workloads then we should
> add a capability check IMO but this looks like a decent foundation to
> me.

the inode_owner_or_capable/inode_permission helpers already do allow
quite a few capabilities there


-- 
Dominique