Re: [PATCH v2 7/7] iomap: Copy larger chunks from userspace

Yin Fengwei
2023-06-07T06:40:11Z

On 6/7/23 02:07, Matthew Wilcox wrote:
> On Mon, Jun 05, 2023 at 04:25:22PM +0800, Yin, Fengwei wrote:
>> On 6/5/2023 6:11 AM, Matthew Wilcox wrote:
>>> On Sun, Jun 04, 2023 at 11:29:52AM -0700, Darrick J. Wong wrote:
>>>> On Fri, Jun 02, 2023 at 11:24:44PM +0100, Matthew Wilcox (Oracle) wrote:
>>>>> - copied = copy_page_from_iter_atomic(page, offset, bytes, i);
>>>>> + copied = copy_page_from_iter_atomic(&folio->page, offset, bytes, i);
>>>>
>>>> I think I've gotten lost in the weeds. Does copy_page_from_iter_atomic
>>>> actually know how to deal with a multipage folio? AFAICT it takes a
>>>> page, kmaps it, and copies @bytes starting at @offset in the page. If
>>>> a caller feeds it a multipage folio, does that all work correctly? Or
>>>> will the pagecache split multipage folios as needed to make it work
>>>> right?
>>>
>>> It's a smidgen inefficient, but it does work. First, it calls
>>> page_copy_sane() to check that offset & n fit within the compound page
>>> (ie this all predates folios).
>>>
>>> ... Oh. copy_page_from_iter() handles this correctly.
>>> copy_page_from_iter_atomic() doesn't. I'll have to fix this
>>> first. Looks like Al fixed copy_page_from_iter() in c03f05f183cd
>>> and didn't fix copy_page_from_iter_atomic().
>>>
>>>> If we create a 64k folio at pos 0 and then want to write a byte at pos
>>>> 40k, does __filemap_get_folio break up the 64k folio so that the folio
>>>> returned by iomap_get_folio starts at 40k? Or can the iter code handle
>>>> jumping ten pages into a 16-page folio and I just can't see it?
>>>
>>> Well ... it handles it fine unless it's highmem. p is kaddr + offset,
>>> so if offset is 40k, it works correctly on !highmem.
>> So is it better to have implementations for !highmem and highmem? And for
>> !highmem, we don't need the kmap_local_page()/kunmap_local() and chunk
>> size per copy is not limited to PAGE_SIZE. Thanks.
>
> No, that's not needed; we can handle that just fine. Maybe this can
> use kmap_local_page() instead of kmap_atomic(). Al, what do you think?
> I haven't tested this yet; need to figure out a qemu config with highmem ...
>
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 960223ed9199..d3d6a0789625 100644
> --- a/lib/iov_iter.c
> +++ 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);
Should be PAGE_SIZE - offset instead of PAGE_SIZE?

> + 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;
> + offset += n;
> + page += offset / PAGE_SIZE;
> + offset %= PAGE_SIZE;
> + n = min_t(size_t, bytes - copied, PAGE_SIZE);

Should be PAGE_SIZE - offset instead of PAGE_SIZE? Thanks.

Regards
Yin, Fengwei

> }
> - 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;
> + return copied;
> }
> EXPORT_SYMBOL(copy_page_from_iter_atomic);
>