linux-kernel - Re: [PATCH] Fix zero_user_segments() with start

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Date:   Fri, 26 Feb 2021 20:51:39 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Fix zero_user_segments() with start > end

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@...r.kernel.org>
> Signed-off-by: OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>

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