linux-kernel - Re: [PATCH 00/14] Small step toward KSM for file back page.

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Date:   Wed, 7 Oct 2020 23:09:16 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Tejun Heo <tj@...nel.org>, Jan Kara <jack@...e.cz>,
        Josef Bacik <jbacik@...com>
Subject: Re: [PATCH 00/14] Small step toward KSM for file back page.

On Wed, Oct 07, 2020 at 01:54:19PM -0400, Jerome Glisse wrote:
> > For other things (NUMA distribution), we can point to something which
> > isn't a struct page and can be distiguished from a real struct page by a
> > bit somewhere (I have ideas for at least three bits in struct page that
> > could be used for this).  Then use a pointer in that data structure to
> > point to the real page.  Or do NUMA distribution at the inode level.
> > Have a way to get from (inode, node) to an address_space which contains
> > just regular pages.
> 
> How do you find all the copies ? KSM maintains a list for a reasons.
> Same would be needed here because if you want to break the write prot
> you need to find all the copy first. If you intend to walk page table
> then how do you synchronize to avoid more copy to spawn while you
> walk reverse mapping, we could lock the struct page i guess. Also how
> do you walk device page table which are completely hidden from core mm.

So ... why don't you put a PageKsm page in the page cache?  That way you
can share code with the current KSM implementation.  You'd need
something like this:

+++ b/mm/filemap.c
@@ -1622,6 +1622,9 @@ struct page *find_lock_entry(struct address_space *mapping
, pgoff_t index)
                lock_page(page);
                /* Has the page been truncated? */
                if (unlikely(page->mapping != mapping)) {
+                       if (PageKsm(page)) {
+                               ...
+                       }
                        unlock_page(page);
                        put_page(page);
                        goto repeat;
@@ -1655,6 +1658,7 @@ struct page *find_lock_entry(struct address_space *mapping, pgoff_t index)
  * * %FGP_WRITE - The page will be written
  * * %FGP_NOFS - __GFP_FS will get cleared in gfp mask
  * * %FGP_NOWAIT - Don't get blocked by page lock
+ * * %FGP_KSM - Return KSM pages
  *
  * If %FGP_LOCK or %FGP_CREAT are specified then the function may sleep even
  * if the %GFP flags specified for %FGP_CREAT are atomic.
@@ -1687,6 +1691,11 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
 
                /* Has the page been truncated? */
                if (unlikely(page->mapping != mapping)) {
+                       if (PageKsm(page) {
+                               if (fgp_flags & FGP_KSM)
+                                       return page;
+                               ...
+                       }
                        unlock_page(page);
                        put_page(page);
                        goto repeat;

I don't know what you want to do when you find a KSM page, so I just left
an ellipsis.