linux-kernel - Re: [PATCH] prctl_set_mm: downgrade mmap

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Date:   Thu, 18 Apr 2019 21:23:21 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     mhocko@...nel.org, akpm@...ux-foundation.org,
        arunks@...eaurora.org, brgl@...ev.pl, geert+renesas@...der.be,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        mguzik@...hat.com, rppt@...ux.ibm.com, vbabka@...e.cz,
        Laurent Dufour <ldufour@...ux.ibm.com>
Subject: Re: [PATCH] prctl_set_mm: downgrade mmap_sem to read lock

On Thu, Apr 18, 2019 at 03:50:39PM +0200, Michal Koutný wrote:
> I learnt, it's, alas, too late to drop the non PRCTL_SET_MM_MAP calls
> [1], so at least downgrade the write acquisition of mmap_sem as in the
> patch below (that should be stacked on the previous one or squashed).
> 
> Cyrill, you mentioned lock changes in [1] but the link seems empty. Is
> it supposed to be [2]? That could be an alternative to this patch after
> some refreshments and clarifications.
> 
> 
> [1] https://lore.kernel.org/lkml/20190417165632.GC3040@uranus.lan/
> [2] https://lore.kernel.org/lkml/20180507075606.870903028@gmail.com/
> 
> ========
> 
> Since commit 88aa7cc688d4 ("mm: introduce arg_lock to protect
> arg_start|end and env_start|end in mm_struct") we use arg_lock for
> boundaries modifications. Synchronize prctl_set_mm with this lock and
> keep mmap_sem for reading only (analogous to what we already do in
> prctl_set_mm_map).
> 
> Also, save few cycles by looking up VMA only after performing basic
> arguments validation.
> 
> Signed-off-by: Michal Koutný <mkoutny@...e.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@...il.com>

As Laurent mentioned we might move vma lookup before the spinlock,
but this might be done on top of the series.