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Date:   Tue, 11 Jul 2023 01:03:18 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Luis Chamberlain <mcgrof@kernel.org>
Cc:     linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org,
        Wang Yugui <wangyugui@e16-tech.com>,
        Dave Chinner <david@fromorbit.com>,
        Christoph Hellwig <hch@infradead.org>,
        "Darrick J . Wong" <djwong@kernel.org>,
        Kent Overstreet <kent.overstreet@linux.dev>
Subject: Re: [PATCH v4 1/9] iov_iter: Handle compound highmem pages in
 copy_page_from_iter_atomic()
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On Mon, Jul 10, 2023 at 04:43:35PM -0700, Luis Chamberlain wrote:
> > -	}
> > -	iterate_and_advance(i, bytes, base, len, off,
> > -		copyin(p + off, base, len),
> > -		memcpy_from_iter(i, p + off, base, len)
> > -	)
> > -	kunmap_atomic(kaddr);
> > -	return bytes;
> > +
> > +	do {
> > +		char *p;
> > +
> > +		n = bytes - copied;
> > +		if (PageHighMem(page)) {
> > +			page += offset / PAGE_SIZE;
> 
> I don't quite follow here how before the page was not modified
> to get to the first kmap_atomic(page) and now immediately if we're
> on a PageHighMem(page) we're doing some arithmetic to the page
> address to get the first kmap_atomic(). The only thing I could
> think of is seems like an implicit assumption here that if its a compound
> highmem page then we always start off with offset with a value of
> 0, is that right? But that seems to not be correct either.

The important thing to know is that it was buggy before.  If you called
copy_page_from_iter_atomic() with an offset larger than PAGE_SIZE, it
corrupted random memory!  I can only assume that nobody was doing that.