Re: [f2fs-dev] [PATCH 1/3] f2fs: call flush_dcache_page when the page was updated

[Changman Lee <cm224.lee@samsung.com>]

Hi Simon,

Thanks very much for your interest.
It becomes more clear due to your explanation.

Regards,
Changman

On Tue, Nov 25, 2014 at 08:05:23PM +0100, Simon Baatz wrote:
> Hi Changman,
> 
> On Mon, Nov 24, 2014 at 11:46:46AM +0900, Changman Lee wrote:
> > Hi Simon,
> > Thanks for your explanation kindly.
> > 
> > On Sun, Nov 23, 2014 at 11:08:54AM +0100, Simon Baatz wrote:
> > > Hi Changman, Jaegeuk,
> > > 
> > > On Thu, Nov 20, 2014 at 05:47:29PM +0900, Changman Lee wrote:
> > > > On Wed, Nov 19, 2014 at 10:45:33PM -0800, Jaegeuk Kim wrote:
> > > > > On Thu, Nov 20, 2014 at 03:04:10PM +0900, Changman Lee wrote:
> > > > > > Hi Jaegeuk,
> > > > > > 
> > > > > > We should call flush_dcache_page before kunmap because the purpose of the cache flush is to address aliasing problem related to virtual address.
> > > > > 
> > > > > Oh, I just followed zero_user_segments below.
> > > > > 
> > > > > static inline void zero_user_segments(struct page *page,
> > > > > 	unsigned start1, unsigned end1,
> > > > > 	unsigned start2, unsigned end2)
> > > > > {
> > > > > 	void *kaddr = kmap_atomic(page);
> > > > > 
> > > > > 	BUG_ON(end1 > PAGE_SIZE || end2 > PAGE_SIZE);
> > > > > 
> > > > > 	if (end1 > start1)
> > > > > 		memset(kaddr + start1, 0, end1 - start1);
> > > > > 
> > > > > 	if (end2 > start2)
> > > > > 		memset(kaddr + start2, 0, end2 - start2);
> > > > > 
> > > > > 	kunmap_atomic(kaddr);
> > > > > 	flush_dcache_page(page);
> > > > > }
> > > > > 
> > > > > Is this a wrong reference? Or, a bug?
> > > > > 
> > > > 
> > > > Well.. Data in cache only have to be flushed until before other users read the data.
> > > > If so, it's not a bug.
> > > > 
> > > 
> > > Yes, it is not a bug, since flush_dcache_page() needs to be able to
> > > deal with non-kmapped pages. However, this may create overhead in
> > > some situations.
> > > 
> > 
> > Previously, I was vague but I thought that it should be different
> > according to vaddr exists or not. So I told jaegeuk that it should
> > be better to change an order between flush_dache_page and kunmap.
> > But actually, it doesn't matter the order between them except
> > the situation you said.
> > Could you explain the situation that makes overhead by flushing after kummap.
> > I can't imagine it by just seeing flush_dcache_page code.
> > 
> 
> I was a not very precise here. Yes, flush_dcache_page() on ARM does
> the same in both situations since it has no idea whether it is called
> before or after kunmap.  However, flush_kernel_dcache_page() can
> assume that it is called before kunmap and thus, for example, does not
> need to pin a highmem page by kmap_high_get() (apart from not having
> to care about flushing user space mappings)
> 
> > > According to documentation (see Documentation/cachetlb.txt), this is
> > > a use for flush_kernel_dcache_page(), since the page has been
> > > modified by the kernel only.  In contrast to flush_dcache_page(),
> > > this function must be called before kunmap().
> > > 
> > > flush_kernel_dcache_page() does not need to flush the user space
> > > aliases.  Additionally, at least on ARM, it does not flush at all
> > > when called within kmap_atomic()/kunmap_atomic(), when
> > > kunmap_atomic() is going to flush the page anyway.  (I know that
> > > almost no one uses flush_kernel_dcache_page() (probably because
> > > almost no one knows when to use which of the two functions), but it
> > > may save a few cache flushes on architectures which are affected by
> > > aliasing)
> > > 
> > > 
> > > > > Anyway I modified as below.
> > > > > 
> > > > > Thanks,
> > > > > 
> > > > > >From 7cb7b27c8cd2efc8a31d79239bef5b41c6e79216 Mon Sep 17 00:00:00 2001
> > > > > From: Jaegeuk Kim <jaegeuk@kernel.org>
> > > > > Date: Tue, 18 Nov 2014 10:50:21 -0800
> > > > > Subject: [PATCH] f2fs: call flush_dcache_page when the page was updated
> > > > > 
> > > > > Whenever f2fs updates mapped pages, it needs to call flush_dcache_page.
> > > > > 
> > > > > Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
> > > > > ---
> > > > >  fs/f2fs/dir.c    | 7 ++++++-
> > > > >  fs/f2fs/inline.c | 2 ++
> > > > >  2 files changed, 8 insertions(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
> > > > > index 5a49995..fabf4ee 100644
> > > > > --- a/fs/f2fs/dir.c
> > > > > +++ b/fs/f2fs/dir.c
> > > > > @@ -287,8 +287,10 @@ void f2fs_set_link(struct inode *dir, struct f2fs_dir_entry *de,
> > > > >  	f2fs_wait_on_page_writeback(page, type);
> > > > >  	de->ino = cpu_to_le32(inode->i_ino);
> > > > >  	set_de_type(de, inode);
> > > > > -	if (!f2fs_has_inline_dentry(dir))
> > > > > +	if (!f2fs_has_inline_dentry(dir)) {
> > > > > +		flush_dcache_page(page);
> > > > >  		kunmap(page);
> > > > > +	}
> > > 
> > > Is this a page that may be mapped into user space? (I may be
> > > completely wrong here, since I have no idea how this code works.  But
> > > it looks like as if the answer is "no" ;-) ).
> > > 
> > > It is not necessary to flush pages that cannot be seen by user space
> > > (see also the NOTE in the documentation of flush_dcache_page() in
> > > cachetlb.txt). Thus, if you know that a page will not be mapped into
> > > user space, please don't create the overhead of flushing it.
> > > 
> > 
> > In the case of dentry unlike inline data, this is not mapped to user space, so dcache flush
> > makes overhead. Do you mean that?
> 
> Yes. I suppose most architectures where D-cache aliasing is an issue
> have optimizations that "defer" the actual flush when there are no
> user space mappings.  But nevertheless, if you already now that there
> can't be any aliases, there is no need to call these functions at
> all.
> 
> 
> - Simon