Adding Marek & Tomasz, On Wed, Jul 30, 2014 at 4:44 PM, Gioh Kim <gioh.kim@xxxxxxx> wrote: > > > 2014-07-22 오후 6:38, Jan Kara 쓴 글: > >> On Tue 22-07-14 09:30:05, Peter Zijlstra wrote: >>> >>> On Tue, Jul 22, 2014 at 02:18:47PM +0900, Gioh Kim wrote: >>>> >>>> Hello, >>>> >>>> This patch try to solve problem that a long-lasting page cache of >>>> ext4 superblock disturbs page migration. >>>> >>>> I've been testing CMA feature on my ARM-based platform >>>> and found some pages for page caches cannot be migrated. >>>> Some of them are page caches of superblock of ext4 filesystem. >>>> >>>> Current ext4 reads superblock with sb_bread(). sb_bread() allocates page >>>> from movable area. But the problem is that ext4 hold the page until >>>> it is unmounted. If root filesystem is ext4 the page cannot be migrated >>>> forever. >>>> >>>> I introduce a new API for allocating page from non-movable area. >>>> It is useful for ext4 and others that want to hold page cache for a long >>>> time. >>> >>> >>> There's no word on why you can't teach ext4 to still migrate that page. >>> For all I know it might be impossible, but at least mention why. > > > I am very sorry for lacking of details. > > In ext4_fill_super() the buffer-head of superblock is stored in sbi->s_sbh. > The page belongs to the buffer-head is allocated from movable area. > To migrate the page the buffer-head should be released via brelse(). > But brelse() is not called until unmount. > > For example, fat_fill_super() reads superblock via sb_bread() > and release it via brelse() immediately. Therefore the page that stores > superblock can be migrated. > > > > >> It doesn't seem to be worth the effort to make that page movable to me >> (it's reasonably doable since superblock buffer isn't accessed in *that* >> many places but single movable page doesn't seem like a good tradeoff for >> the complexity). >> >> But this made me look into the migration code and it isn't completely >> clear >> to me what makes the migration code decide that sb buffer isn't movable? >> We >> seem to be locking the buffers before moving the underlying page but we >> don't do any reference or state checks on the buffers... That seems to be >> assuming that noone looks at bh->b_data without holding buffer lock. That >> is likely true for ordinary data but definitely not true for metadata >> buffers (i.e., buffers for pages from block device mappings). > we got similar issues and add similar work-around codes. Thank you, Kyungmin Park > > The sb buffer is not movable because it is not released. > sb_bread increase the reference counter of buffer-head so that > the page of the buffer-head cannot be movable. > > sb_bread allocates page from movable area but it is not movable until the > reference counter of the buffer-head becomes zero. > There is no lock for the buffer but the reference counter acts like lock. > > Actually it is strange that ext4 keeps buffer-head in superblock structure > until unmount (it can be long time) > I thinks the buffer-head should be released immediately like > fat_fill_super() did. > I believe there is a reason to keep buffer-head so that I suggest this > patch. > > > > >> >> Added linux-mm to CC to enlighten me a bit ;) >> >> Honza >> > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
