Re: [patch 2/13] remove pages from the LRU in __free_pages_ok()

Andrew Morton
2002-07-29T00:21:06Z

Linus Torvalds wrote:
>
> On Sun, 28 Jul 2002, Andrew Morton wrote:
> >
> > There are some situations where a page's final release is performed by
> > put_page(). Such as in access_process_vm(). This tends to go BUG()
> > because the page is on the LRU.
>
> This is wrong.
>
> If that happens, then you should just make access_process_vm() use
> "page_cache_release()". That's what you basically make "__free_pages_ok()"
> do, but since you still have to add the BUG_ON() check to make clear that
> it is illegal to do this from an interrupt context, it's much better to
> just do this check statically.
>

OK. This means that put_page() against userspace-mapped pages
is to be avoided.

Did an audit. What on earth is drivers/scsi/sg.c:sg_rb_correct4mmap()
doing?

Also skb_release_data(), ___pskb_trim() and __pskb_pull_tail(). Can these
ever perform the final release against a page which is on the LRU? In
interrupt context?

tcp_sendmsg() is doing put_page() too. I _think_ it's OK because
the page is mapped into the calling process and because of the
way in which shrink_cache() looks at page->count. Not sure.

fs/binfmt_elf.c | 3 ++-
fs/smbfs/file.c | 8 ++++----
kernel/ptrace.c | 3 ++-
3 files changed, 8 insertions(+), 6 deletions(-)

--- 2.5.29/fs/binfmt_elf.c~put_page Sun Jul 28 17:17:23 2002
+++ 2.5.29-akpm/fs/binfmt_elf.c Sun Jul 28 17:31:33 2002
@@ -33,6 +33,7 @@
#include <linux/smp_lock.h>
#include <linux/compiler.h>
#include <linux/highmem.h>
+#include <linux/pagemap.h>

#include <asm/uaccess.h>
#include <asm/param.h>
@@ -1249,7 +1250,7 @@ static int elf_core_dump(long signr, str
flush_page_to_ram(page);
kunmap(page);
}
- put_page(page);
+ page_cache_release(page);
}
}
}
--- 2.5.29/fs/smbfs/file.c~put_page Sun Jul 28 17:19:08 2002
+++ 2.5.29-akpm/fs/smbfs/file.c Sun Jul 28 17:19:40 2002
@@ -105,9 +105,9 @@ smb_readpage(struct file *file, struct p
int error;
struct dentry *dentry = file->f_dentry;

- get_page(page);
+ page_cache_get(page);
error = smb_readpage_sync(dentry, page);
- put_page(page);
+ page_cache_release(page);
return error;
}

@@ -194,11 +194,11 @@ smb_writepage(struct page *page)
if (page->index >= end_index+1 || !offset)
return -EIO;
do_it:
- get_page(page);
+ page_cache_get(page);
err = smb_writepage_sync(inode, page, 0, offset);
SetPageUptodate(page);
unlock_page(page);
- put_page(page);
+ page_cache_release(page);
return err;
}

--- 2.5.29/kernel/ptrace.c~put_page Sun Jul 28 17:20:04 2002
+++ 2.5.29-akpm/kernel/ptrace.c Sun Jul 28 17:26:13 2002
@@ -11,6 +11,7 @@
#include <linux/errno.h>
#include <linux/mm.h>
#include <linux/highmem.h>
+#include <linux/pagemap.h>
#include <linux/smp_lock.h>

#include <asm/pgtable.h>
@@ -156,7 +157,7 @@ int access_process_vm(struct task_struct
flush_page_to_ram(page);
}
kunmap(page);
- put_page(page);
+ page_cache_release(page);
len -= bytes;
buf += bytes;
addr += bytes;

.