Re: asynchronous readahead prefetcher operation

[Matthew Wilcox <willy@infradead.org>]

On Wed, Jul 12, 2017 at 11:31:21PM +0300, Vasilis Dimitsas wrote:
> I am currently working on a project which is related to the operation of
> the linux readahead prefetcher. As a result, I am trying to understand its
> operation. Having read thoroughly the relevant part in the kernel code, I
> realize, from the comments, that part of the prefetching occurs
> asynchronously. The problem is that I can not verify this from the code.
> 
> Even if you call page_cache_sync_readahead() or
> page_cache_async_readahead(), then both will end up in ra_submit(), in
> which, the operation is common for both cases.
> 
> So, please could you tell me at which point does the operation of
> prefetching occurs asynchronously?

The prefetching operation always occurs asynchronously; the
I/O is submitted and then both page_cache_sync_readahead() and
page_cache_async_readahead() return to the caller.  They use slightly
different algorithms, which is why they're different functions, but the
I/O is not waited for.  It's up to the caller to do that.

I imagine you're looking at filemap_fault(), and it happens like this:

        page = find_get_page(mapping, offset);
(returns NULL because there's no page in the cache)
                do_sync_mmap_readahead(vmf->vma, ra, file, offset);
(will create pages and put them in the page cache, taking PageLock on each page)
                page = find_get_page(mapping, offset);
(finds the page that was just created)
        if (!lock_page_or_retry(page, vmf->vma->vm_mm, vmf->flags)) {
(will attempt to lock the page ... if it's locked and the fault lets us retry,
fails so we can handle retries at the higher level.  If it's locked and the
fault says we can't retry, then sleeps until unlocked.  If/once it's unlocked,
will return success)

When the I/O completes, the page will be unlocked, usually by calling
page_endio().

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