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Date: Sun, 22 May 2011 15:25:31 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
cc: Christoph Hellwig <hch@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Adding an ugliness in __read_cache_page()?
Linus, Christoph,
Could you bear a patch something like the one below?
I have a series aimed at 2.6.41 to remove mm/shmem.c's peculiar radix
tree of swap entries, using slots in the file's standard radix_tree
instead - prompted in part by https://lkml.org/lkml/2011/1/22/110
In isolating the occurrence of swap entries in radix_trees, it becomes
convenient to remove shmem's ->readpage at last, giving it instead its
own ->splice_read.
But drivers/gpu/drm i915 and ttm are using read_cache_page_gfp() or
read_mapping_page() on tmpfs: on objects created by shmem_file_setup().
Nothing else uses read_cache_page_gfp(). I cannot find anything else
using read_mapping_page() on tmpfs, but wonder if something might be
out there. Stacked filesystems appear not to go that way nowadays.
Would it be better to make i915 and ttm call shmem_read_cache_page()
directly? Perhaps removing the then unused read_cache_page_gfp(), or
perhaps not: may still be needed for i915 and ttm on tiny !SHMEM ramfs.
I find both ways ugly, but no nice alternative: introducing a new method
when the known callers are already tied to tmpfs/ramfs seems over the top.
Thanks,
Hugh
---
include/linux/mm.h | 1 +
mm/filemap.c | 13 +++++++++++++
mm/shmem.c | 26 +++++++++++++++-----------
3 files changed, 29 insertions(+), 11 deletions(-)
--- 2.6.39/include/linux/mm.h 2011-05-18 21:06:34.000000000 -0700
+++ linux/include/linux/mm.h 2011-05-22 11:50:49.332431949 -0700
@@ -873,6 +873,7 @@ extern void __show_free_areas(unsigned i
int shmem_lock(struct file *file, int lock, struct user_struct *user);
struct file *shmem_file_setup(const char *name, loff_t size, unsigned long flags);
int shmem_zero_setup(struct vm_area_struct *);
+struct page *shmem_read_cache_page(struct address_space *, pgoff_t, gfp_t);
#ifndef CONFIG_MMU
extern unsigned long shmem_get_unmapped_area(struct file *file,
--- 2.6.39/mm/filemap.c 2011-05-18 21:06:34.000000000 -0700
+++ linux/mm/filemap.c 2011-05-22 11:50:49.332431949 -0700
@@ -1762,6 +1762,19 @@ static struct page *__read_cache_page(st
{
struct page *page;
int err;
+
+#ifdef CONFIG_TMPFS
+ /*
+ * The ->readpage() interface does not suit tmpfs at all, since it
+ * may have pages in swapcache, and needs to find those for itself;
+ * but gpu/drm/i915 and gpu/drm/ttm need it to support this function.
+ */
+ if (!filler) {
+ BUG_ON(!mapping_cap_swap_backed(mapping));
+ return shmem_read_cache_page(mapping, index, gfp);
+ }
+#endif
+
repeat:
page = find_get_page(mapping, index);
if (!page) {
--- 2.6.39/mm/shmem.c 2011-05-18 21:06:34.000000000 -0700
+++ linux/mm/shmem.c 2011-05-22 11:50:49.332431949 -0700
@@ -1652,17 +1652,22 @@ static struct inode *shmem_get_inode(str
static const struct inode_operations shmem_symlink_inode_operations;
static const struct inode_operations shmem_symlink_inline_operations;
-/*
- * Normally tmpfs avoids the use of shmem_readpage and shmem_write_begin;
- * but providing them allows a tmpfs file to be used for splice, sendfile, and
- * below the loop driver, in the generic fashion that many filesystems support.
- */
-static int shmem_readpage(struct file *file, struct page *page)
+struct page *
+shmem_read_cache_page(struct address_space *mapping, pgoff_t index, gfp_t gfp)
{
- struct inode *inode = page->mapping->host;
- int error = shmem_getpage(inode, page->index, &page, SGP_CACHE, NULL);
- unlock_page(page);
- return error;
+ struct page *page;
+ int error;
+
+ /*
+ * Not shown: addition of gfp arg to shmem_getpage(), passed down here.
+ * Not shown: addition of shmem_file_splice_read() avoiding ->readpage.
+ */
+ error = shmem_getpage(mapping->host, index, &page, SGP_CACHE, NULL);
+ if (error)
+ page = ERR_PTR(error);
+ else
+ unlock_page(page);
+ return page;
}
static int
@@ -2475,7 +2480,6 @@ static const struct address_space_operat
.writepage = shmem_writepage,
.set_page_dirty = __set_page_dirty_no_writeback,
#ifdef CONFIG_TMPFS
- .readpage = shmem_readpage,
.write_begin = shmem_write_begin,
.write_end = shmem_write_end,
#endif
--
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