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ab4443fe3c
ra_alloc_folio() marks a page that should trigger next round of async readahead. However it rounds up computed index to the order of page being allocated. This can however lead to multiple consecutive pages being marked with readahead flag. Consider situation with index == 1, mark == 1, order == 0. We insert order 0 page at index 1 and mark it. Then we bump order to 1, index to 2, mark (still == 1) is rounded up to 2 so page at index 2 is marked as well. Then we bump order to 2, index is incremented to 4, mark gets rounded to 4 so page at index 4 is marked as well. The fact that multiple pages get marked within a single readahead window confuses the readahead logic and results in readahead window being trimmed back to 1. This situation is triggered in particular when maximum readahead window size is not a power of two (in the observed case it was 768 KB) and as a result sequential read throughput suffers. Fix the problem by rounding 'mark' down instead of up. Because the index is naturally aligned to 'order', we are guaranteed 'rounded mark' == index iff 'mark' is within the page we are allocating at 'index' and thus exactly one page is marked with readahead flag as required by the readahead code and sequential read performance is restored. This effectively reverts part of commit |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.