Re: [LSF/MM/BPF TOPIC] Cloud storage optimizations

[Pankaj Raghav <p.raghav@xxxxxxxxxxx>]

On 2023-03-16 16:29, Pankaj Raghav wrote:
> Hi Keith,
> 
> On 2023-03-03 23:32, Keith Busch wrote:
>>> Yes, clearly it says *yet* so that begs the question what would be
>>> required?
>>
>> Oh, gotcha. I'll work on a list of places it currently crashes.
>>  
> I started looking into this to see why it crashes when we increase the LBA
> size of a block device greater than the page size. These are my primary
> findings:
> 
> - Block device aops (address_space_operations) are all based on buffer
> head, which limits us to work on only PAGE_SIZE chunks.
> 
> For a 8k LBA size, the stack trace you posted ultimately fails inside
> alloc_page_buffers as the size will be > PAGE_SIZE.
> 
> struct buffer_head *alloc_page_buffers(struct page *page, unsigned long
> size, bool retry)
> 
Aghh. Sorry for the ugly formatting:

struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
		bool retry)
{
	struct buffer_head *bh, *head;
	gfp_t gfp = GFP_NOFS | __GFP_ACCOUNT;
	long offset;
	struct mem_cgroup *memcg, *old_memcg;

	if (retry)
		gfp |= __GFP_NOFAIL;

	/* The page lock pins the memcg */
	memcg = page_memcg(page);
	old_memcg = set_active_memcg(memcg);

	head = NULL;
	offset = PAGE_SIZE;
	while ((offset -= size) >= 0) {
	// we will not go into this loop as offset will be negative
	...
	...
	}
...
return head; // we return NULL for LBA size > 4k
}