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Diffstat (limited to 'mm/readahead.c')
| -rw-r--r-- | mm/readahead.c | 99 |
1 files changed, 99 insertions, 0 deletions
diff --git a/mm/readahead.c b/mm/readahead.c index cf0dcf89eb69..73b2bc5302e0 100644 --- a/mm/readahead.c +++ b/mm/readahead.c @@ -8,6 +8,105 @@ * Initial version. */ +/** + * DOC: Readahead Overview + * + * Readahead is used to read content into the page cache before it is + * explicitly requested by the application. Readahead only ever + * attempts to read pages that are not yet in the page cache. If a + * page is present but not up-to-date, readahead will not try to read + * it. In that case a simple ->readpage() will be requested. + * + * Readahead is triggered when an application read request (whether a + * systemcall or a page fault) finds that the requested page is not in + * the page cache, or that it is in the page cache and has the + * %PG_readahead flag set. This flag indicates that the page was loaded + * as part of a previous read-ahead request and now that it has been + * accessed, it is time for the next read-ahead. + * + * Each readahead request is partly synchronous read, and partly async + * read-ahead. This is reflected in the struct file_ra_state which + * contains ->size being to total number of pages, and ->async_size + * which is the number of pages in the async section. The first page in + * this async section will have %PG_readahead set as a trigger for a + * subsequent read ahead. Once a series of sequential reads has been + * established, there should be no need for a synchronous component and + * all read ahead request will be fully asynchronous. + * + * When either of the triggers causes a readahead, three numbers need to + * be determined: the start of the region, the size of the region, and + * the size of the async tail. + * + * The start of the region is simply the first page address at or after + * the accessed address, which is not currently populated in the page + * cache. This is found with a simple search in the page cache. + * + * The size of the async tail is determined by subtracting the size that + * was explicitly requested from the determined request size, unless + * this would be less than zero - then zero is used. NOTE THIS + * CALCULATION IS WRONG WHEN THE START OF THE REGION IS NOT THE ACCESSED + * PAGE. + * + * The size of the region is normally determined from the size of the + * previous readahead which loaded the preceding pages. This may be + * discovered from the struct file_ra_state for simple sequential reads, + * or from examining the state of the page cache when multiple + * sequential reads are interleaved. Specifically: where the readahead + * was triggered by the %PG_readahead flag, the size of the previous + * readahead is assumed to be the number of pages from the triggering + * page to the start of the new readahead. In these cases, the size of + * the previous readahead is scaled, often doubled, for the new + * readahead, though see get_next_ra_size() for details. + * + * If the size of the previous read cannot be determined, the number of + * preceding pages in the page cache is used to estimate the size of + * a previous read. This estimate could easily be misled by random + * reads being coincidentally adjacent, so it is ignored unless it is + * larger than the current request, and it is not scaled up, unless it + * is at the start of file. + * + * In general read ahead is accelerated at the start of the file, as + * reads from there are often sequential. There are other minor + * adjustments to the read ahead size in various special cases and these + * are best discovered by reading the code. + * + * The above calculation determines the readahead, to which any requested + * read size may be added. + * + * Readahead requests are sent to the filesystem using the ->readahead() + * address space operation, for which mpage_readahead() is a canonical + * implementation. ->readahead() should normally initiate reads on all + * pages, but may fail to read any or all pages without causing an IO + * error. The page cache reading code will issue a ->readpage() request + * for any page which ->readahead() does not provided, and only an error + * from this will be final. + * + * ->readahead() will generally call readahead_page() repeatedly to get + * each page from those prepared for read ahead. It may fail to read a + * page by: + * + * * not calling readahead_page() sufficiently many times, effectively + * ignoring some pages, as might be appropriate if the path to + * storage is congested. + * + * * failing to actually submit a read request for a given page, + * possibly due to insufficient resources, or + * + * * getting an error during subsequent processing of a request. + * + * In the last two cases, the page should be unlocked to indicate that + * the read attempt has failed. In the first case the page will be + * unlocked by the caller. + * + * Those pages not in the final ``async_size`` of the request should be + * considered to be important and ->readahead() should not fail them due + * to congestion or temporary resource unavailability, but should wait + * for necessary resources (e.g. memory or indexing information) to + * become available. Pages in the final ``async_size`` may be + * considered less urgent and failure to read them is more acceptable. + * They will eventually be read individually using ->readpage(). + */ + #include <linux/kernel.h> #include <linux/dax.h> #include <linux/gfp.h> |