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Date:   Mon, 07 Dec 2020 21:13:31 +0800
From:   Chengguang Xu <cgxu519@mykernel.net>
Reply-To: cgxu519@mykernel.net
To:     "Dominique Martinet" <asmadeus@codewreck.org>
Cc:     "ericvh" <ericvh@gmail.com>, "lucho" <lucho@ionkov.net>,
        "linux-fsdevel" <linux-fsdevel@vger.kernel.org>,
        "linux-kernel" <linux-kernel@vger.kernel.org>,
        "v9fs-developer" <v9fs-developer@lists.sourceforge.net>
Message-ID: <1763d55f12b.11abdd4431975.7848752990749710617@mykernel.net>
In-Reply-To: <20201207112410.GA26628@nautica>
References: <20201205130904.518104-1-cgxu519@mykernel.net>
 <20201206091618.GA22629@nautica>
 <20201206205318.GA25257@nautica>
 <1763bcb5b8e.da1e98e51195.9022463261101254548@mykernel.net> <20201207112410.GA26628@nautica>
Subject: Re: [V9fs-developer] [RFC PATCH] 9p: create writeback fid on shared
 mmap
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 ---- =E5=9C=A8 =E6=98=9F=E6=9C=9F=E4=B8=80, 2020-12-07 19:24:10 Dominique =
Martinet <asmadeus@codewreck.org> =E6=92=B0=E5=86=99 ----
 > Chengguang Xu wrote on Mon, Dec 07, 2020:
 > >  > , VM_MAYWRITE is set anytime we have a shared map where file has
 > >  > been opened read-write, which seems to be what you want with regard=
s to
 > >  > protecting from mprotect calls.
 > >  >=20
 > >  > How about simply changing check from WRITE to MAYWRITE?
 > >=20
 > > It would be fine and based on the code in do_mmap(), it  seems we even=
 don't
 > > need extra check here.  The condition (vma->vm_flags & VM_SHARED) will=
 be enough.
 > > Am I missing something?
 >=20
 > VM_MAYWRITE is unset if the file hasn't been open for writing (in which
 > case the mapping can't be mprotect()ed to writable map), so checking it
 > is a bit more efficient.
 >=20
 > Anyway I'd like to obsolete the writeback fid uses now that fids have a
 > refcount (this usecase can be a simple refcount increase), in which case
 > efficiency is less of a problem, but we're not there yet...
 >=20
 > Please resend with MAYWRITE if you want authorship and I'll try to take
 > some time to test incl. the mprotect usecase.
 >=20

Thanks for the review, I'll send revised version later.

Thanks,
Chengguang