From: Yin Fengwei <fengwei.yin@intel.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: "Darrick J. Wong" <djwong@kernel.org>,
<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>,
Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v2 7/7] iomap: Copy larger chunks from userspace
Date: Thu, 8 Jun 2023 09:22:09 +0800 [thread overview]
Message-ID: <8fa8decd-82ca-43cd-e94c-ebfc1c2782b5@intel.com> (raw)
In-Reply-To: <ZICoXy5TEgSy0yFr@casper.infradead.org>
On 6/7/23 23:55, Matthew Wilcox wrote:
> On Wed, Jun 07, 2023 at 10:21:41AM +0800, Yin Fengwei wrote:
>> On 6/7/23 02:07, Matthew Wilcox wrote:
>>> +++ b/lib/iov_iter.c
>>> @@ -857,24 +857,36 @@ size_t iov_iter_zero(size_t bytes, struct iov_iter *i)
>>> }
>>> EXPORT_SYMBOL(iov_iter_zero);
>>>
>>> -size_t copy_page_from_iter_atomic(struct page *page, unsigned offset, size_t bytes,
>>> - struct iov_iter *i)
>>> +size_t copy_page_from_iter_atomic(struct page *page, unsigned offset,
>>> + size_t bytes, struct iov_iter *i)
>>> {
>>> - char *kaddr = kmap_atomic(page), *p = kaddr + offset;
>>> - if (!page_copy_sane(page, offset, bytes)) {
>>> - kunmap_atomic(kaddr);
>>> + size_t n = bytes, copied = 0;
>>> +
>>> + if (!page_copy_sane(page, offset, bytes))
>>> return 0;
>>> - }
>>> - if (WARN_ON_ONCE(!i->data_source)) {
>>> - kunmap_atomic(kaddr);
>>> + if (WARN_ON_ONCE(!i->data_source))
>>> return 0;
>>> +
>>> + page += offset / PAGE_SIZE;
>>> + offset %= PAGE_SIZE;
>>> + if (PageHighMem(page))
>>> + n = min_t(size_t, bytes, PAGE_SIZE);
>> This is smart.
>
> Thanks ;-)
>
>>> + while (1) {
>>> + char *kaddr = kmap_atomic(page) + offset;
>>> + iterate_and_advance(i, n, base, len, off,
>>> + copyin(kaddr + off, base, len),
>>> + memcpy_from_iter(i, kaddr + off, base, len)
>>> + )
>>> + kunmap_atomic(kaddr);
>>> + copied += n;
>>> + if (!PageHighMem(page) || copied == bytes || n == 0)
>>> + break;
>> My understanding is copied == bytes could cover !PageHighMem(page).
>
> It could! But the PageHighMem test serves two purposes. One is that
> it tells the human reader that this is all because of HighMem. The
> other is that on !HIGHMEM systems it compiles to false:
>
> PAGEFLAG_FALSE(HighMem, highmem)
>
> static inline int Page##uname(const struct page *page) { return 0; }
>
> So it tells the _compiler_ that all of this code is ignorable and
> it can optimise it out. Now, you and I know that it will always
> be true, but it lets the compiler remove the test. Hopefully the
> compiler can also see that:
>
> while (1) {
> ...
> if (true)
> break;
> }
>
> means it can optimise away the entire loop structure and just produce
> the same code that it always did.
I thought about the first purpose. But the second purpose is new thing
I learned from this thread. Thanks a lot for detail explanation.
Regards
Yin, Fengwei
next prev parent reply other threads:[~2023-06-08 1:23 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-06-02 22:24 [PATCH v2 0/7] Create large folios in iomap buffered write path Matthew Wilcox (Oracle)
2023-06-02 22:24 ` [PATCH v2 1/7] iomap: Remove large folio handling in iomap_invalidate_folio() Matthew Wilcox (Oracle)
2023-06-04 17:58 ` Darrick J. Wong
2023-06-05 7:11 ` Christoph Hellwig
2023-06-02 22:24 ` [PATCH v2 2/7] doc: Correct the description of ->release_folio Matthew Wilcox (Oracle)
2023-06-04 17:55 ` Darrick J. Wong
2023-06-04 20:10 ` Matthew Wilcox
2023-06-04 20:33 ` Darrick J. Wong
2023-06-05 13:11 ` Matthew Wilcox
2023-06-05 15:07 ` Darrick J. Wong
2023-06-05 7:12 ` Christoph Hellwig
2023-06-02 22:24 ` [PATCH v2 3/7] iomap: Remove unnecessary test from iomap_release_folio() Matthew Wilcox (Oracle)
2023-06-04 18:01 ` Darrick J. Wong
2023-06-04 21:39 ` Matthew Wilcox
2023-06-05 21:10 ` Ritesh Harjani
2023-06-05 7:13 ` Christoph Hellwig
2023-06-02 22:24 ` [PATCH v2 4/7] filemap: Add fgp_t typedef Matthew Wilcox (Oracle)
2023-06-04 18:02 ` Darrick J. Wong
2023-06-05 7:14 ` Christoph Hellwig
2023-06-02 22:24 ` [PATCH v2 5/7] filemap: Allow __filemap_get_folio to allocate large folios Matthew Wilcox (Oracle)
2023-06-04 18:09 ` Darrick J. Wong
2023-06-04 21:48 ` Matthew Wilcox
2023-06-05 15:21 ` Darrick J. Wong
2023-06-05 7:16 ` Christoph Hellwig
2023-06-02 22:24 ` [PATCH v2 6/7] iomap: Create large folios in the buffered write path Matthew Wilcox (Oracle)
2023-06-04 18:10 ` Darrick J. Wong
2023-06-05 7:16 ` Christoph Hellwig
2023-06-02 22:24 ` [PATCH v2 7/7] iomap: Copy larger chunks from userspace Matthew Wilcox (Oracle)
2023-06-04 18:29 ` Darrick J. Wong
2023-06-04 22:11 ` Matthew Wilcox
2023-06-05 8:25 ` Yin, Fengwei
2023-06-06 18:07 ` Matthew Wilcox
2023-06-07 2:21 ` Yin Fengwei
2023-06-07 5:33 ` Yin, Fengwei
2023-06-07 15:55 ` Matthew Wilcox
2023-06-08 1:22 ` Yin Fengwei [this message]
2023-06-07 6:40 ` Yin Fengwei
2023-06-07 15:56 ` Matthew Wilcox
2023-06-04 0:19 ` [PATCH v2 0/7] Create large folios in iomap buffered write path Wang Yugui
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