Commit da6052f7 authored by Nick Piggin's avatar Nick Piggin Committed by Linus Torvalds
Browse files

[PATCH] update some mm/ comments



Let's try to keep mm/ comments more useful and up to date. This is a start.
Signed-off-by: default avatarNick Piggin <npiggin@suse.de>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent e5ac9c5a
......@@ -219,7 +219,8 @@ struct inode;
* Each physical page in the system has a struct page associated with
* it to keep track of whatever it is we are using the page for at the
* moment. Note that we have no way to track which tasks are using
* a page.
* a page, though if it is a pagecache page, rmap structures can tell us
* who is mapping it.
*/
struct page {
unsigned long flags; /* Atomic flags, some possibly
......@@ -299,8 +300,7 @@ struct page {
*/
/*
* Drop a ref, return true if the logical refcount fell to zero (the page has
* no users)
* Drop a ref, return true if the refcount fell to zero (the page has no users)
*/
static inline int put_page_testzero(struct page *page)
{
......@@ -356,43 +356,55 @@ void split_page(struct page *page, unsigned int order);
* For the non-reserved pages, page_count(page) denotes a reference count.
* page_count() == 0 means the page is free. page->lru is then used for
* freelist management in the buddy allocator.
* page_count() == 1 means the page is used for exactly one purpose
* (e.g. a private data page of one process).
* page_count() > 0 means the page has been allocated.
*
* A page may be used for kmalloc() or anyone else who does a
* __get_free_page(). In this case the page_count() is at least 1, and
* all other fields are unused but should be 0 or NULL. The
* management of this page is the responsibility of the one who uses
* it.
* Pages are allocated by the slab allocator in order to provide memory
* to kmalloc and kmem_cache_alloc. In this case, the management of the
* page, and the fields in 'struct page' are the responsibility of mm/slab.c
* unless a particular usage is carefully commented. (the responsibility of
* freeing the kmalloc memory is the caller's, of course).
*
* The other pages (we may call them "process pages") are completely
* A page may be used by anyone else who does a __get_free_page().
* In this case, page_count still tracks the references, and should only
* be used through the normal accessor functions. The top bits of page->flags
* and page->virtual store page management information, but all other fields
* are unused and could be used privately, carefully. The management of this
* page is the responsibility of the one who allocated it, and those who have
* subsequently been given references to it.
*
* The other pages (we may call them "pagecache pages") are completely
* managed by the Linux memory manager: I/O, buffers, swapping etc.
* The following discussion applies only to them.
*
* A page may belong to an inode's memory mapping. In this case,
* page->mapping is the pointer to the inode, and page->index is the
* file offset of the page, in units of PAGE_CACHE_SIZE.
* A pagecache page contains an opaque `private' member, which belongs to the
* page's address_space. Usually, this is the address of a circular list of
* the page's disk buffers. PG_private must be set to tell the VM to call
* into the filesystem to release these pages.
*
* A page contains an opaque `private' member, which belongs to the
* page's address_space. Usually, this is the address of a circular
* list of the page's disk buffers.
* A page may belong to an inode's memory mapping. In this case, page->mapping
* is the pointer to the inode, and page->index is the file offset of the page,
* in units of PAGE_CACHE_SIZE.
*
* For pages belonging to inodes, the page_count() is the number of
* attaches, plus 1 if `private' contains something, plus one for
* the page cache itself.
* If pagecache pages are not associated with an inode, they are said to be
* anonymous pages. These may become associated with the swapcache, and in that
* case PG_swapcache is set, and page->private is an offset into the swapcache.
*
* Instead of keeping dirty/clean pages in per address-space lists, we instead
* now tag pages as dirty/under writeback in the radix tree.
* In either case (swapcache or inode backed), the pagecache itself holds one
* reference to the page. Setting PG_private should also increment the
* refcount. The each user mapping also has a reference to the page.
*
* There is also a per-mapping radix tree mapping index to the page
* in memory if present. The tree is rooted at mapping->root.
* The pagecache pages are stored in a per-mapping radix tree, which is
* rooted at mapping->page_tree, and indexed by offset.
* Where 2.4 and early 2.6 kernels kept dirty/clean pages in per-address_space
* lists, we instead now tag pages as dirty/writeback in the radix tree.
*
* All process pages can do I/O:
* All pagecache pages may be subject to I/O:
* - inode pages may need to be read from disk,
* - inode pages which have been modified and are MAP_SHARED may need
* to be written to disk,
* - private pages which have been modified may need to be swapped out
* to swap space and (later) to be read back into memory.
* to be written back to the inode on disk,
* - anonymous pages (including MAP_PRIVATE file mappings) which have been
* modified may need to be swapped out to swap space and (later) to be read
* back into memory.
*/
/*
......
......@@ -13,24 +13,25 @@
* PG_reserved is set for special pages, which can never be swapped out. Some
* of them might not even exist (eg empty_bad_page)...
*
* The PG_private bitflag is set if page->private contains a valid value.
* The PG_private bitflag is set on pagecache pages if they contain filesystem
* specific data (which is normally at page->private). It can be used by
* private allocations for its own usage.
*
* During disk I/O, PG_locked is used. This bit is set before I/O and
* reset when I/O completes. page_waitqueue(page) is a wait queue of all tasks
* waiting for the I/O on this page to complete.
* During initiation of disk I/O, PG_locked is set. This bit is set before I/O
* and cleared when writeback _starts_ or when read _completes_. PG_writeback
* is set before writeback starts and cleared when it finishes.
*
* PG_locked also pins a page in pagecache, and blocks truncation of the file
* while it is held.
*
* page_waitqueue(page) is a wait queue of all tasks waiting for the page
* to become unlocked.
*
* PG_uptodate tells whether the page's contents is valid. When a read
* completes, the page becomes uptodate, unless a disk I/O error happened.
*
* For choosing which pages to swap out, inode pages carry a PG_referenced bit,
* which is set any time the system accesses that page through the (mapping,
* index) hash table. This referenced bit, together with the referenced bit
* in the page tables, is used to manipulate page->age and move the page across
* the active, inactive_dirty and inactive_clean lists.
*
* Note that the referenced bit, the page->lru list_head and the active,
* inactive_dirty and inactive_clean lists are protected by the
* zone->lru_lock, and *NOT* by the usual PG_locked bit!
* PG_referenced, PG_reclaim are used for page reclaim for anonymous and
* file-backed pagecache (see mm/vmscan.c).
*
* PG_error is set to indicate that an I/O error occurred on this page.
*
......@@ -42,6 +43,10 @@
* space, they need to be kmapped separately for doing IO on the pages. The
* struct page (these bits with information) are always mapped into kernel
* address space...
*
* PG_buddy is set to indicate that the page is free and in the buddy system
* (see mm/page_alloc.c).
*
*/
/*
......@@ -74,7 +79,7 @@
#define PG_checked 8 /* kill me in 2.5.<early>. */
#define PG_arch_1 9
#define PG_reserved 10
#define PG_private 11 /* Has something at ->private */
#define PG_private 11 /* If pagecache, has fs-private data */
#define PG_writeback 12 /* Page is under writeback */
#define PG_nosave 13 /* Used for system suspend/resume */
......@@ -83,7 +88,7 @@
#define PG_mappedtodisk 16 /* Has blocks allocated on-disk */
#define PG_reclaim 17 /* To be reclaimed asap */
#define PG_nosave_free 18 /* Free, should not be written */
#define PG_nosave_free 18 /* Used for system suspend/resume */
#define PG_buddy 19 /* Page is free, on buddy lists */
......
......@@ -599,8 +599,8 @@ void fastcall __lock_page_nosync(struct page *page)
* @mapping: the address_space to search
* @offset: the page index
*
* A rather lightweight function, finding and getting a reference to a
* hashed page atomically.
* Is there a pagecache struct page at the given (mapping, offset) tuple?
* If yes, increment its refcount and return it; if no, return NULL.
*/
struct page * find_get_page(struct address_space *mapping, unsigned long offset)
{
......@@ -987,7 +987,7 @@ void do_generic_mapping_read(struct address_space *mapping,
/* Get exclusive access to the page ... */
lock_page(page);
/* Did it get unhashed before we got the lock? */
/* Did it get truncated before we got the lock? */
if (!page->mapping) {
unlock_page(page);
page_cache_release(page);
......@@ -1627,7 +1627,7 @@ static struct page * filemap_getpage(struct file *file, unsigned long pgoff,
page_not_uptodate:
lock_page(page);
/* Did it get unhashed while we waited for it? */
/* Did it get truncated while we waited for it? */
if (!page->mapping) {
unlock_page(page);
goto err;
......
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment