linux-kernel - Re: [V9fs-developer] [RFC PATCH] 9p: create writeback fid on shared mmap

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Date:   Mon, 07 Dec 2020 21:13:31 +0800
From:   Chengguang Xu <cgxu519@...ernel.net>
To:     "Dominique Martinet" <asmadeus@...ewreck.org>
Cc:     "ericvh" <ericvh@...il.com>, "lucho" <lucho@...kov.net>,
        "linux-fsdevel" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel" <linux-kernel@...r.kernel.org>,
        "v9fs-developer" <v9fs-developer@...ts.sourceforge.net>
Subject: Re: [V9fs-developer] [RFC PATCH] 9p: create writeback fid on shared
 mmap

 ---- 在 星期一, 2020-12-07 19:24:10 Dominique Martinet <asmadeus@...ewreck.org> 撰写 ----
 > 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 regards to
 > >  > protecting from mprotect calls.
 > >  > 
 > >  > 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 enough.
 > > Am I missing something?
 > 
 > 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.
 > 
 > 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...
 > 
 > Please resend with MAYWRITE if you want authorship and I'll try to take
 > some time to test incl. the mprotect usecase.
 > 

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

Thanks,
Chengguang