PAGE_CACHE_PREV_HOLE(9) Memory Management in Linux PAGE_CACHE_PREV_HOLE(9)NAME
page_cache_prev_hole - find the prev hole (not-present entry)
pgoff_t page_cache_prev_hole(struct address_space * mapping, pgoff_t index, unsigned long max_scan);
maximum range to search
Search backwards in the range [max(index-max_scan+1, 0), index] for the first hole.
the index of the hole if found, otherwise returns an index outside of the set specified (in which case 'index - return >= max_scan' will be
true). In rare cases of wrap-around, ULONG_MAX will be returned.
page_cache_prev_hole may be called under rcu_read_lock. However, like radix_tree_gang_lookup, this will not atomically search a snapshot of
the tree at a single point in time. For example, if a hole is created at index 10, then subsequently a hole is created at index 5,
page_cache_prev_hole covering both indexes may return 5 if called under rcu_read_lock.
COPYRIGHT Kernel Hackers Manual 3.10 June 2014 PAGE_CACHE_PREV_HOLE(9)
Check Out this Related Man Page
MPAGE_READPAGES(9) The Linux VFS MPAGE_READPAGES(9)NAME
mpage_readpages - populate an address space with some pages & start reads against them
int mpage_readpages(struct address_space * mapping, struct list_head * pages, unsigned nr_pages, get_block_t get_block);
The address of a list_head which contains the target pages. These pages have their ->index populated and are otherwise uninitialised.
The page at pages->prev has the lowest file offset, and reads should be issued in pages->prev to pages->next order.
The number of pages at *pages
The filesystem's block mapper function.
This function walks the pages and the blocks within each page, building and emitting large BIOs.
If anything unusual happens, such as:
- encountering a page which has buffers - encountering a page which has a non-hole after a hole - encountering a page with non-contiguous
then this code just gives up and calls the buffer_head-based read function. It does handle a page which has holes at the end - that is a
common case: the end-of-file on blocksize < PAGE_CACHE_SIZE setups.
There is a problem. The mpage read code assembles several pages, gets all their disk mappings, and then submits them all. That's fine, but
obtaining the disk mappings may require I/O. Reads of indirect blocks, for example.
So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
SUBMITTED IN THE FOLLOWING ORDER
12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
because the indirect block has to be read to get the mappings of blocks 13,14,15,16. Obviously, this impacts performance.
So what we do it to allow the filesystem's get_block function to set BH_Boundary when it maps block 11. BH_Boundary says: mapping of the
block after this one will require I/O against a block which is probably close to this one. So you should push what I/O you have currently
This all causes the disk requests to be issued in the correct order.
COPYRIGHT Kernel Hackers Manual 3.10 June 2014 MPAGE_READPAGES(9)
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