The future of readahead

Both Kent and David have had conversations with me about improving the readahead filesystem interface this last week, and as I don't have time to write the code, here's the design.

1. Kent doesn't like it that we do an XArray lookup for each page. The proposed solution adds a (small) array of page pointers (or a pagevec) to the struct readahead_control. It may make sense to move __readahead_batch() and readahead_page() out of line at that point. This should be backed up with performance numbers.

2. David wants to be sure that readahead is aligned to a granule size (eg 256kB) to support fscache. When we last talked about it, I suggested encoding the granule size in the struct address_space. I no longer think this approach should be pursued, since ...

3. Kent wants to be able to expand readahead to encompass an entire fs extent (if, eg, that extent is compressed or encrypted). We don't know that at the right point; the filesystem can't pass that information through the generic_file_buffered_read() or filemap_fault() interface to the readahead code. So the right approach here is for the filesystem to ask the readahead code to expand the readahead batch.

So solving #2 and #3 looks like a new interface for filesystems to call:

void readahead_expand(struct readahead_control *rac, loff_t start, u64 len); or possibly
void readahead_expand(struct readahead_control *rac, pgoff_t start,

		unsigned int count);

It might not actually expand the readahead attempt at all -- for example, if there's already a page in the page cache, or if it can't allocate memory. But this puts the responsibility for allocating pages in the VFS, where it belongs.

4. Mike wants to be able to do 4MB I/Os [1]. That should be covered by the solution above. Mike, just to clarify. Do you need 4MB pages, or can you work with some mixture of page sizes going as far as 1024 x 4kB pages?

5. I'm allocating larger pages in the readahead code (part of the THP patch set [2])

[1] https://lore.kernel.org/linux-fsdevel/CAOg9mSSrJp2dqQTNDgucLoeQcE_E_aYPxnRe5xphhdSPYw7QtQ@mail.gmail.com/
[2] http://git.infradead.org/users/willy/pagecache.git/commitdiff/c00bd4082c7bc32a17b0baa29af6974286978e1f

This is exactly what the fscache read helper in my fscache rewrite is doing, except that I'm doing it in fs/fscache/read_helper.c.

Have a look here:

	https://lore.kernel.org/linux-fsdevel/159465810864.1376674.10267227421160756746.stgit@warthog.procyon.org.uk/

and look for the fscache_read_helper() function.

Note that it's slighly complicated because it handles ->readpage(), ->readpages() and ->write_begin()[*].

[*] I want to be able to bring the granule into the cache for modification.
Ideally I'd be able to see that the entire granule is going to get written over and skip - kind of like write_begin for a whole granule rather than a page.

Shaping the readahead request has the following issues:

(1) The request may span multiple granules.

(2) Those granules may be a mixture of cached and uncached.

(3) The granule size may vary.

(4) Granules fall on power-of-2 boundaries (for example 256K boundaries) within the file, but the request may not start on a boundary and may not end on one.

To deal with this, fscache_read_helper() calls out to the cache backend (fscache_shape_request()) and the netfs (req->ops->reshape()) to adjust the read it's going to make. Shaping the request may mean moving the start earlier as well as expanding or contracting the size. The only thing that's guaranteed is that the first page of the request will be retained.

I also don't let a request cross a cached/uncached boundary, but rather cut the request off there and return. The filesystem can then generate a new request and call back in. (Note that I have to be able to keep track of the filesystem's metadata so that I can reissue the request to the netfs in the event that cache suffers some sort of error).

What I was originally envisioning for the new ->readahead() interface is add a second aop that allows the shaping to be accessed by the VM, before it's started pinning any pages.

The shaping parameters I think we need are:

- The inode, for i_size and fscache cookie
- The proposed page range

and what you would get back could be:

- Shaped page range
- Minimum I/O granularity[1]
- Minimum preferred granularity[2]
- Flag indicating if the pages can just be zero-filled[3]

[1] The filesystem doesn't want to read in smaller chunks than this.

[2] The cache doesn't want to read in smaller chunks than this, though in the cache's case, a partially read block is just abandoned for the moment. This number would allow the readahead algorithm to shorten the request if it can't allocate a page.

[3] If I know that the local i_size is much bigger than the i_size on the server, there's no need to download/read those pages and readahead can just clear them. This is more applicable to write_begin() normally.

Now a chunk of this is in struct readahead_control, so it might be reasonable to add the other bits there too.

Note that one thing I really would like to avoid having to do is to expand a request forward, particularly if the main page of interest is precreated and locked by the VM before calling the filesystem. I would much rather the VM created the pages, starting from the lowest-numbered.

Anyway, that's my 2p.
David

I'm going to want something like that for THP too. I may end up changing the write_begin API.

Thank you for illustrating why this shouldn't be handled in the filesystem ;-)

mmap readaround starts n/2 pages below the the faulting page and ends n/2 pages above the faulting page. So if your adjustment fails to actually bring in the page in the middel, it has failed mmap.

A call to ->readahead is always a contiguous set of pages. A call to readahead_expand() which tried to expand both up and down would start by allocating, locking and adding the pages to the page cache heading downwards from the current start (it's usually not allowed to lock pages out of order, but because they're locked before being added, this is an exception). Then we'd try to expand upwards. We'll fail to expand if we can't allocate a page or if there's already a page in the cache that blocks our expansion in that direction.

As far as the zeroing beyond i_size, that's the responsibility of the filesystem.