Re: [PATCH 6/8] btrfs: store a kernel virtual address in struct sector_ptr

[Christoph Hellwig <hch@lst.de>]

On Thu, Apr 10, 2025 at 08:04:25AM +0930, Qu Wenruo wrote:
> 在 2025/4/9 20:40, Christoph Hellwig 写道:
>> All data pointed to by struct sector_ptr is non-highmem kernel memory.
>> Simplify the code by using a void pointer instead of a page + offset
>> pair and dropping all the kmap calls.
>
> But the higher level bio from btrfs filemaps can have highmem pages.
>
> That's why we keep the kmap/kunmap.

Where do filemap pages come into the raid code?   As far as I can see
they are always copied, and the memory is only allocated in the raid
code.  As seen in this code we have two direct allocations that I
convered to __get_free_page, and one case that uses the bulk page
allocator where I use page_address.  Or did I miss something?

> Or is there a way to set the filemap to no use any highmem pages?

You can restrict the allocation of a mapping to avoid highmem using
mapping_set_gfp_mask().  But that would not help with direct I/O IFF
user pages came into this code.