Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 26 Feb 2021 20:51:39 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] Fix zero_user_segments() with start > end
Message-ID: <20210226205139.GI2723601@casper.infradead.org>
References: <87eeh2erm0.fsf@mail.parknet.co.jp>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <87eeh2erm0.fsf@mail.parknet.co.jp>
Precedence: bulk

On Sat, Feb 27, 2021 at 01:11:35AM +0900, OGAWA Hirofumi wrote:
> zero_user_segments() is used from __block_write_begin_int(), for
> example like the following
> 
> 	zero_user_segments(page, 4096, 1024, 512, 918)
> 
> But new zero_user_segments() implements for HIGMEM + TRANSPARENT_HUGEPAGE 
> doesn't handle "start > end" case correctly, and hits BUG_ON(). (we
> can fix __block_write_begin_int() instead though, it is the old and
> multiple usage)

Why don't we just take out the BUG_ON instead?  The function doesn't
actually do the wrong thing.

> Also it calls kmap_atomic() unnecessary while start == end == 0.

I'm OK with that.  It always used to do that.

> Cc: <stable@vger.kernel.org>
> Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>

Fixes: 0060ef3b4e6d ("mm: support THPs in zero_user_segments")