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

Yin, Fengwei
2023-06-07T05:33:51Z

On 6/7/2023 10:21 AM, Yin Fengwei wrote:
>
>
> 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);
> This is smart.
>
>> + 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).
>
>> + offset += n;
>> + page += offset / PAGE_SIZE;
> Should be page += n / PAGE_SIZE? Thanks.
offset / PAGE_SIZE is correct. Sorry for the noise.

Regards
Yin, Fengwei

>
>
> Regards
> Yin, Fengwei
>
>> + offset %= PAGE_SIZE;
>> + n = min_t(size_t, bytes - copied, PAGE_SIZE);
>> }
>> - 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);
>>