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Date:   Mon, 07 Dec 2020 14:02:31 +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: <1763bcb5b8e.da1e98e51195.9022463261101254548@mykernel.net>
In-Reply-To: <20201206205318.GA25257@nautica>
References: <20201205130904.518104-1-cgxu519@mykernel.net>
 <20201206091618.GA22629@nautica> <20201206205318.GA25257@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 04:53:18 Dominique =
Martinet <asmadeus@codewreck.org> =E6=92=B0=E5=86=99 ----
 > Dominique Martinet wrote on Sun, Dec 06, 2020:
 > > Chengguang Xu wrote on Sat, Dec 05, 2020:
 > > > If vma is shared and the file was opened for writing,
 > > > we should also create writeback fid because vma may be
 > > > mprotected writable even if now readonly.
 > >=20
 > > Hm, I guess it makes sense.
 >=20
 > I had a second look, and generic_file_readonly_mmap uses vma's
 > `vma->vm_flags & VM_MAYWRITE` instead (together with VM_SHARED),
 > while mapping_writably_mapped ultimately basically only seems to
 > validate that the mapping is shared from a look at mapping_map_writable
 > callers? It's not very clear to me.
 >=20
 > , 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 regards to
 > protecting from mprotect calls.
 >=20
 > How about simply changing check from WRITE to MAYWRITE?

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 e=
nough.
Am I missing something?

Thanks,
Chengguang

 >=20
 >      v9inode =3D V9FS_I(inode);
 >      mutex_lock(&v9inode->v_mutex);
 >      if (!v9inode->writeback_fid &&
 >          (vma->vm_flags & VM_SHARED) &&
 > -        (vma->vm_flags & VM_WRITE)) {
 > +        (vma->vm_flags & VM_MAYWRITE)) {
 >          /*
 >           * clone a fid and add it to writeback_fid
 >           * we do it during mmap instead of