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Subject[PATCH] tmpfs 3/9 shmem_getpage reading holes
Here I intended a patch to remove the unsatisfactory shmem_recalc_inode
(which tries to work out when vmscan has freed undirtied hole pages,
to relax its blocks-in-use limit; but can only do so per-inode when it
needs to be per-super). I had hoped to use the releasepage method, but
it looks like it can't quite be bent to this task, the page might still
be rebusied after release. 2.4-ac uses a removepage method dedicated to
this, but I'm reluctant to ask for such a minor address_space_operation.

So, leave shmem_recalc_inode as is, but avoid the issue as much as
possible, by letting shmem_getpage use the empty_zero_page instead of
allocating when a hole is read (but this cannot be done when it's being
mapped, nowadays the nopage doesn't know if page will be copied or not).
Whereupon shmem_getpage(,,,SGP_READ) can do partial trunc's holdpage.

--- tmpfs2/mm/shmem.c Thu Oct 17 22:00:59 2002
+++ tmpfs3/mm/shmem.c Thu Oct 17 22:01:09 2002
@@ -51,12 +51,16 @@
/* Keep swapped page count in private field of indirect struct page */
#define nr_swapped private

-/* Flag end-of-file treatment to shmem_getpage and shmem_swp_alloc */
+/* Flag allocation requirements to shmem_getpage and shmem_swp_alloc */
enum sgp_type {
- SGP_READ, /* don't exceed i_size */
- SGP_WRITE, /* may exceed i_size */
+ SGP_READ, /* don't exceed i_size, don't allocate page */
+ SGP_CACHE, /* don't exceed i_size, may allocate page */
+ SGP_WRITE, /* may exceed i_size, may allocate page */
};

+static int shmem_getpage(struct inode *inode, unsigned long idx,
+ struct page **pagep, enum sgp_type sgp);
+
static inline struct page *shmem_dir_alloc(unsigned int gfp_mask)
{
/*
@@ -138,8 +142,7 @@
* @inode: inode to recalc
*
* We have to calculate the free blocks since the mm can drop
- * undirtied hole pages behind our back. Later we should be
- * able to use the releasepage method to handle this better.
+ * undirtied hole pages behind our back.
*
* But normally info->alloced == inode->i_mapping->nrpages + info->swapped
* So mm freed is info->alloced - (inode->i_mapping->nrpages + info->swapped)
@@ -278,7 +281,7 @@
*
* @info: info structure for the inode
* @index: index of the page to find
- * @sgp: check and recheck i_size?
+ * @sgp: check and recheck i_size? skip allocation?
*/
static swp_entry_t *shmem_swp_alloc(struct shmem_inode_info *info, unsigned long index, enum sgp_type sgp)
{
@@ -286,12 +289,15 @@
struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
struct page *page = NULL;
swp_entry_t *entry;
+ static const swp_entry_t unswapped = {0};

if (sgp != SGP_WRITE &&
((loff_t) index << PAGE_CACHE_SHIFT) >= inode->i_size)
return ERR_PTR(-EINVAL);

while (!(entry = shmem_swp_entry(info, index, &page))) {
+ if (sgp == SGP_READ)
+ return (swp_entry_t *) &unswapped;
/*
* Test free_blocks against 1 not 0, since we have 1 data
* page (and perhaps indirect index pages) yet to allocate:
@@ -483,7 +489,6 @@

static int shmem_notify_change(struct dentry *dentry, struct iattr *attr)
{
- static struct page *shmem_holdpage(struct inode *, unsigned long);
struct inode *inode = dentry->d_inode;
struct page *page = NULL;
long change = 0;
@@ -508,8 +513,9 @@
* it assigned to swap.
*/
if (attr->ia_size & (PAGE_CACHE_SIZE-1)) {
- page = shmem_holdpage(inode,
- attr->ia_size >> PAGE_CACHE_SHIFT);
+ (void) shmem_getpage(inode,
+ attr->ia_size>>PAGE_CACHE_SHIFT,
+ &page, SGP_READ);
}
}
}
@@ -812,6 +818,16 @@
shmem_swp_unmap(entry);
spin_unlock(&info->lock);
swap_free(swap);
+ } else if (sgp == SGP_READ) {
+ shmem_swp_unmap(entry);
+ page = find_get_page(mapping, idx);
+ if (page && TestSetPageLocked(page)) {
+ spin_unlock(&info->lock);
+ wait_on_page_locked(page);
+ page_cache_release(page);
+ goto repeat;
+ }
+ spin_unlock(&info->lock);
} else {
shmem_swp_unmap(entry);
spin_unlock(&info->lock);
@@ -854,40 +870,14 @@
SetPageUptodate(page);
}

- /* We have the page */
- unlock_page(page);
- *pagep = page;
+ if (page) {
+ unlock_page(page);
+ *pagep = page;
+ } else
+ *pagep = ZERO_PAGE(0);
return 0;
}

-static struct page *shmem_holdpage(struct inode *inode, unsigned long idx)
-{
- struct shmem_inode_info *info = SHMEM_I(inode);
- struct page *page;
- swp_entry_t *entry;
- swp_entry_t swap = {0};
-
- /*
- * Somehow, it feels wrong for truncation down to cause any
- * allocation: so instead of a blind shmem_getpage, check that
- * the page has actually been instantiated before holding it.
- */
- spin_lock(&info->lock);
- page = find_get_page(inode->i_mapping, idx);
- if (!page) {
- entry = shmem_swp_entry(info, idx, NULL);
- if (entry) {
- swap = *entry;
- shmem_swp_unmap(entry);
- }
- }
- spin_unlock(&info->lock);
- if (swap.val) {
- (void) shmem_getpage(inode, idx, &page, SGP_READ);
- }
- return page;
-}
-
struct page *shmem_nopage(struct vm_area_struct *vma, unsigned long address, int unused)
{
struct inode *inode = vma->vm_file->f_dentry->d_inode;
@@ -899,7 +889,7 @@
idx += vma->vm_pgoff;
idx >>= PAGE_CACHE_SHIFT - PAGE_SHIFT;

- error = shmem_getpage(inode, idx, &page, SGP_READ);
+ error = shmem_getpage(inode, idx, &page, SGP_CACHE);
if (error)
return (error == -ENOMEM)? NOPAGE_OOM: NOPAGE_SIGBUS;

@@ -1191,7 +1181,8 @@
}
nr -= offset;

- if (!list_empty(&mapping->i_mmap_shared))
+ if (!list_empty(&mapping->i_mmap_shared) &&
+ page != ZERO_PAGE(0))
flush_dcache_page(page);

/*
-
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