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SubjectRe: [rfc][patch] mm, fs: warn on missing address space operations
On 03/22/2010 07:39 AM, Nick Piggin wrote:
> It's ugly and lazy that we do these default aops in case it has not
> been filled in by the filesystem.
>
> A NULL operation should always mean either: we don't support the
> operation; we don't require any action; or a bug in the filesystem,
> depending on the context.
>
> In practice, if we get rid of these fallbacks, it will be clearer
> what operations are used by a given address_space_operations struct,
> reduce branches, reduce #if BLOCK ifdefs, and should allow us to get
> rid of all the buffer_head knowledge from core mm and fs code.
>
> We could add a patch like this which spits out a recipe for how to fix
> up filesystems and get them all converted quite easily.
>

Guilty as charged. It could be nice if the first thing this patch will
do is: Add a comment at struct address_space_operations that states
that all operations should be defined and perhaps what are the minimum
defaults for some of these, as below. I do try to be good, I was under
the impression that defaults are OK.

Below are changes to exofs as proposed by this mail. Please comment?
I will push such a patch to exofs later.

> Index: linux-2.6/mm/filemap.c
> ===================================================================
> --- linux-2.6.orig/mm/filemap.c
> +++ linux-2.6/mm/filemap.c
> @@ -2472,7 +2472,14 @@ int try_to_release_page(struct page *pag
>
> if (mapping && mapping->a_ops->releasepage)
> return mapping->a_ops->releasepage(page, gfp_mask);
> - return try_to_free_buffers(page);
> + else {
> + static bool warned = false;
> + if (!warned) {
> + warned = true;
> + print_symbol("address_space_operations %s missing releasepage method. Use try_to_free_buffers.\n", (unsigned long)page->mapping->a_ops);
> + }
> + return try_to_free_buffers(page);

This is not nice! we should have an export that looks like:

int default_releasepage(struct page *page, gfp_t gfp)
{
return try_to_free_buffers(page);
}
otherwise 3ton FSes would have to define the same thing all over again

> + }
> }
>
> EXPORT_SYMBOL(try_to_release_page);
> Index: linux-2.6/mm/page-writeback.c
> ===================================================================
> --- linux-2.6.orig/mm/page-writeback.c
> +++ linux-2.6/mm/page-writeback.c
> @@ -1159,8 +1159,14 @@ int set_page_dirty(struct page *page)
> if (likely(mapping)) {
> int (*spd)(struct page *) = mapping->a_ops->set_page_dirty;
> #ifdef CONFIG_BLOCK
> - if (!spd)
> + if (!spd) {
> + static bool warned = false;
> + if (!warned) {
> + warned = true;
> + print_symbol("address_space_operations %s missing set_page_dirty method. Use __set_page_dirty_buffers.\n", (unsigned long)page->mapping->a_ops);
> + }
> spd = __set_page_dirty_buffers;
> + }

We could do better then __set_page_dirty_buffers for something lots of FSes use

> #endif
> return (*spd)(page);
> }
> Index: linux-2.6/mm/truncate.c
> ===================================================================
> --- linux-2.6.orig/mm/truncate.c
> +++ linux-2.6/mm/truncate.c
> @@ -16,8 +16,8 @@
> #include <linux/highmem.h>
> #include <linux/pagevec.h>
> #include <linux/task_io_accounting_ops.h>
> -#include <linux/buffer_head.h> /* grr. try_to_release_page,
> - do_invalidatepage */
> +#include <linux/buffer_head.h> /* block_invalidatepage */
> +#include <linux/kallsyms.h>
> #include "internal.h"
>
>
> @@ -40,8 +40,14 @@ void do_invalidatepage(struct page *page
> void (*invalidatepage)(struct page *, unsigned long);
> invalidatepage = page->mapping->a_ops->invalidatepage;
> #ifdef CONFIG_BLOCK
> - if (!invalidatepage)
> + if (!invalidatepage) {
> + static bool warned = false;
> + if (!warned) {
> + warned = true;
> + print_symbol("address_space_operations %s missing invalidatepage method. Use block_invalidatepage.\n", (unsigned long)page->mapping->a_ops);
> + }
> invalidatepage = block_invalidatepage;
> + }
> #endif
> if (invalidatepage)
> (*invalidatepage)(page, offset);
> Index: linux-2.6/fs/inode.c
> ===================================================================
> --- linux-2.6.orig/fs/inode.c
> +++ linux-2.6/fs/inode.c
> @@ -25,6 +25,7 @@
> #include <linux/mount.h>
> #include <linux/async.h>
> #include <linux/posix_acl.h>
> +#include <linux/kallsyms.h>
>
> /*
> * This is needed for the following functions:
> @@ -311,8 +312,14 @@ void clear_inode(struct inode *inode)
>
> might_sleep();
> /* XXX: filesystems should invalidate this before calling */
> - if (!mapping->a_ops->release)
> + if (!mapping->a_ops->release) {

Where is this code from? As of 2.6.34-rc2 I don't have a release in a_ops->

> + static bool warned = false;
> + if (!warned) {
> + warned = true;
> + print_symbol("address_space_operations %s missing release method. Use invalidate_inode_buffers.\n", (unsigned long)mapping->a_ops);
> + }
> invalidate_inode_buffers(inode);
> + }
>
> BUG_ON(mapping_has_private(mapping));
> BUG_ON(mapping->nrpages);
> @@ -393,9 +400,14 @@ static int invalidate_list(struct list_h
> if (inode->i_state & I_NEW)
> continue;
> mapping = &inode->i_data;
> - if (!mapping->a_ops->release)
> + if (!mapping->a_ops->release) {

And here

> + static bool warned = false;
> + if (!warned) {
> + warned = true;
> + print_symbol("address_space_operations %s missing release method. Using invalidate_inode_buffers.\n", (unsigned long)mapping->a_ops);
> + }
> invalidate_inode_buffers(inode);
> - else
> + } else
> mapping->a_ops->release(mapping, AOP_RELEASE_FORCE);
> BUG_ON(mapping_has_private(mapping));
> if (!atomic_read(&inode->i_count)) {
> @@ -497,8 +509,14 @@ static void prune_icache(int nr_to_scan)
> spin_unlock(&inode_lock);
> if (mapping->a_ops->release)
> ret = mapping->a_ops->release(mapping, 0);
> - else
> + else {
> + static bool warned = false;
> + if (!warned) {
> + warned = true;
> + print_symbol("address_space_operations %s missing release method. Using remove_inode_buffers.\n", (unsigned long)mapping->a_ops);
> + }
> ret = !remove_inode_buffers(inode);
> + }
> if (ret)
> reap += invalidate_mapping_pages(&inode->i_data,
> 0, -1);
> Index: linux-2.6/fs/libfs.c
> ===================================================================
> --- linux-2.6.orig/fs/libfs.c
> +++ linux-2.6/fs/libfs.c
> @@ -11,6 +11,7 @@
> #include <linux/exportfs.h>
> #include <linux/writeback.h>
> #include <linux/buffer_head.h>
> +#include <linux/kallsyms.h>
>
> #include <asm/uaccess.h>
>
> @@ -827,9 +828,14 @@ int simple_fsync(struct file *file, stru
> int err;
> int ret;
>
> - if (!mapping->a_ops->sync)
> + if (!mapping->a_ops->sync) {

And this one is new as well, right?

New added vectors should be added to all in-tree FSes by the submitter.
and a deprecate warning for out of tree FSes (If we care)

> + static bool warned = false;
> + if (!warned) {
> + warned = true;
> + print_symbol("address_space_operations %s missing sync method. Using sync_mapping_buffers.\n", (unsigned long)mapping->a_ops);
> + }
> ret = sync_mapping_buffers(mapping);
> - else
> + } else
> ret = mapping->a_ops->sync(mapping, AOP_SYNC_WRITE|AOP_SYNC_WAIT);
>
> if (!(inode->i_state & I_DIRTY))
> --
---
exofs: Add default address_space_operations

All vectors of address_space_operations should be initialized by the filesystem.
Add the missing parts.

---
git diff --stat -p -M fs/exofs/inode.c
fs/exofs/inode.c | 8 ++++++++
1 files changed, 8 insertions(+), 0 deletions(-)
diff --git a/fs/exofs/inode.c b/fs/exofs/inode.c
index a17e4b7..85dd847 100644
--- a/fs/exofs/inode.c
+++ b/fs/exofs/inode.c
@@ -754,6 +754,11 @@ static int exofs_write_end(struct file *file, struct address_space *mapping,
return ret;
}

+static int exofs_releasepage(struct page *page, gfp_t gfp)
+{
+ return try_to_free_buffers(page);
+}
+
const struct address_space_operations exofs_aops = {
.readpage = exofs_readpage,
.readpages = exofs_readpages,
@@ -761,6 +766,9 @@ const struct address_space_operations exofs_aops = {
.writepages = exofs_writepages,
.write_begin = exofs_write_begin_export,
.write_end = exofs_write_end,
+ .releasepage = exofs_releasepage,
+ .set_page_dirty = __set_page_dirty_buffers,
+ .invalidatepage = block_invalidatepage,
};

/******************************************************************************

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