Re: [PATCH -next] ovl: respect underlying filesystem's get_unmapped_area()

[Jinjiang Tu <tujinjiang@huawei.com>]

在 2024/12/9 21:33, Matthew Wilcox 写道:
> On Mon, Dec 09, 2024 at 02:43:08PM +0800, Jinjiang Tu wrote:
>> 在 2024/12/6 21:58, Matthew Wilcox 写道:
>>> On Thu, Dec 05, 2024 at 10:30:38PM +0800, Jinjiang Tu wrote:
>>>> During our tests in containers, there is a read-only file (i.e., shared
>>> Show your test.
>> I mmap an overlayfs file with PROT_READ, and call madvise(MADV_COLLAPSE), the code
>> is as follows:
>>
>> 	fd = open(path, O_RDONLY);
>> 	addr = mmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
>> 	ret = madvise(addr, size, MADV_COLLAPSE);
>>
>> The addr isn't THP-aligned and ret is -1, errno is EINVAL.
> Then your test is buggy.
>
>           * Check alignment for file vma and size for both file and anon vma by
>           * filtering out the unsuitable orders.
>
> You didn't align your mmap, so it's expected to fail.

When mmap an ext4 file, since ext4_file_operations defines ".get_unmapped_area = thp_get_unmapped_area", it calls thp_get_unmapped_area() in __get_unmapped_area to
mmap at a THP-aligned address.

For overlayfs file, it's underlying filesystem may be ext4, which supports large folio. For this situation, should we mmap at a THP-aligned address too, to map
THP?

Thanks.