linux-kernel - Re: [PATCH] vmap(): don't allow invalid pages

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Date:   Thu, 20 Jan 2022 04:27:35 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Yury Norov <yury.norov@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Alexey Klimov <aklimov@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Uladzislau Rezki <urezki@...il.com>
Subject: Re: [PATCH] vmap(): don't allow invalid pages

On Thu, Jan 20, 2022 at 09:07:11AM +0530, Anshuman Khandual wrote:
> 
> 
> On 1/19/22 10:52 PM, Yury Norov wrote:
> >> Why should not this just scan over the entire user provided struct page
> >> array and make sure that all pages there in are valid via above method,
> >> but in vmap() itself before calling vmap_pages_range(). Because seems
> >> like a single invalid page detected in vmap_pages_pte_range() will
> >> anyways abort the entire vmap(). This will also enable us to drop the
> >> existing NULL check above.
> >
> > I can do this, but why is it any better than the current approach?
> 
> Because it will just return on the first instance where the valid page
> check fails, saving us some CPU cycles and an incomplete mapping ?

The valid page check is never intended to fail!  If you're worried about
the efficiency of doing this, you have seriously confused your priorities.