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Subject[PATCH] Add PAGE_CACHE_PAGES
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This patch introduces PAGE_CACHE_PAGES which is the number of pages in a
page cache.

Why?

Thus spake mm/shmem.c:

idx = (address - vma->vm_start) >> PAGE_CACHE_SHIFT;
idx += vma->vm_pgoff;

But include/linux/mm.h says:

unsigned long vm_pgoff; /* Offset (within vm_file) in PAGE_SIZE
units, *not* PAGE_CACHE_SIZE */
and include/linux/pagemap.h adds:

* The page cache can done in larger chunks than
* one page, because it allows for more efficient
* throughput (it can then be mapped into user
* space in smaller chunks for same flexibility).
*
* Or rather, it _will_ be done in larger chunks.

I also found two other places in mm/filemap.c that treat vma->vm_pgoff
as counting in PAGE_CACHE_SIZE units.

This patch only makes sense if PAGE_CACHE_SIZE is a multiple of
PAGE_SIZE but it seems logical to me that they will continue to be so
even when they wont be equel as they are now.

Comments are welcome.

Gilad.



diff -Nur -X /home/gby/dontdiff linux-2.4.19/include/linux/pagemap.h linux/include/linux/pagemap.h
--- linux-2.4.19/include/linux/pagemap.h Fri Jun 21 01:38:38 2002
+++ linux/include/linux/pagemap.h Thu Aug 15 21:53:15 2002
@@ -27,6 +27,7 @@
#define PAGE_CACHE_SIZE PAGE_SIZE
#define PAGE_CACHE_MASK PAGE_MASK
#define PAGE_CACHE_ALIGN(addr) (((addr)+PAGE_CACHE_SIZE-1)&PAGE_CACHE_MASK)
+#define PAGE_CACHE_PAGES (PAGE_CACHE_SIZE / PAGE_SIZE)

#define page_cache_get(x) get_page(x)
#define page_cache_release(x) __free_page(x)
diff -Nur -X /home/gby/dontdiff linux-2.4.19/mm/filemap.c linux/mm/filemap.c
--- linux-2.4.19/mm/filemap.c Thu Aug 15 21:31:38 2002
+++ linux/mm/filemap.c Thu Aug 15 21:45:48 2002
@@ -1918,8 +1918,8 @@
struct page *page, **hash;
unsigned long size, pgoff, endoff;

- pgoff = ((address - area->vm_start) >> PAGE_CACHE_SHIFT) + area->vm_pgoff;
- endoff = ((area->vm_end - area->vm_start) >> PAGE_CACHE_SHIFT) + area->vm_pgoff;
+ pgoff = ((address - area->vm_start) >> PAGE_CACHE_SHIFT) + (area->vm_pgoff / PAGE_CACHE_PAGES);
+ endoff = ((area->vm_end - area->vm_start) >> PAGE_CACHE_SHIFT) + (area->vm_pgoff / PAGE_CACHE_PAGES);

retry_all:
/*
diff -Nur -X /home/gby/dontdiff linux-2.4.19/mm/shmem.c linux/mm/shmem.c
--- linux-2.4.19/mm/shmem.c Thu Aug 15 21:31:38 2002
+++ linux/mm/shmem.c Thu Aug 15 21:41:21 2002
@@ -663,7 +663,7 @@
struct inode * inode = vma->vm_file->f_dentry->d_inode;

idx = (address - vma->vm_start) >> PAGE_CACHE_SHIFT;
- idx += vma->vm_pgoff;
+ idx += (vma->vm_pgoff / PAGE_CACHE_PAGES);

if (shmem_getpage(inode, idx, &page))
return page;

--
Gilad Ben-Yossef <gilad@benyossef.com>
http://benyossef.com

"Money talks, bullshit walks and GNU awks."
-- Shachar "Sun" Shemesh, debt collector for the GNU/Yakuza

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