TAG_PAGES_FOR_WRITEB(9) Memory Management in Linux TAG_PAGES_FOR_WRITEB(9)NAME
tag_pages_for_writeback - tag pages to be written by write_cache_pages
SYNOPSIS
void tag_pages_for_writeback(struct address_space * mapping, pgoff_t start, pgoff_t end);
ARGUMENTS
mapping
address space structure to write
start
starting page index
end
ending page index (inclusive)
DESCRIPTION
This function scans the page range from start to end (inclusive) and tags all pages that have DIRTY tag set with a special TOWRITE tag. The
idea is that write_cache_pages (or whoever calls this function) will then use TOWRITE tag to identify pages eligible for writeback. This
mechanism is used to avoid livelocking of writeback by a process steadily creating new dirty pages in the file (thus it is important for
this function to be quick so that it can tag pages faster than a dirtying process can create them).
COPYRIGHT Kernel Hackers Manual 3.10 June 2014 TAG_PAGES_FOR_WRITEB(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
SYNOPSIS
int mpage_readpages(struct address_space * mapping, struct list_head * pages, unsigned nr_pages, get_block_t get_block);
ARGUMENTS
mapping
the address_space
pages
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.
nr_pages
The number of pages at *pages
get_block
The filesystem's block mapper function.
DESCRIPTION
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
blocks
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.
BH_BOUNDARY EXPLANATION
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
accumulated.
This all causes the disk requests to be issued in the correct order.
COPYRIGHT Kernel Hackers Manual 2.6. July 2010 MPAGE_READPAGES(9)
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