On 11/03/2015 04:49 AM, Hugh Dickins wrote: > On Mon, 2 Nov 2015, Boaz Harrosh wrote: >> On 11/02/2015 01:39 AM, Hugh Dickins wrote: >> <> >>>> This patch is not correct! >>> >>> I think you have actually confirmed that the patch is correct: >>> why bother to test PageDirty or PageWriteback when PageUptodate >>> already tells you what you need? >>> >>> Or do these filesystems do something unusual with PageUptodate >>> when PageDirty is set? I didn't find it. >>> >> >> This is kind of delicate stuff. It took me a while to get it right >> when I did it. I don't remember all the details. >> >> But consider this option: > > Thanks, yes, it helps to have a concrete example in front of us. > >> >> exofs_write_begin on a full PAGE_CACHE_SIZE, the page is instantiated >> new in page-cache is that PageUptodate(page) then? I thought not. > > Right, PageUptodate must not be set until the page has been filled with > the correct data. Nor is PageDirty or PageWriteback set at this point, > actually. > > Once page is filled with the correct data, either exofs_write_end() > (which uses simple_write_end()) or (internally) exofs_commit_chunk() > is called. > >> (exofs does not set that) > > It's simple_write_end() or exofs_commit_chunk() which SetPageUptodate > in this case. And after that each calls set_page_dirty(), which does > the SetPageDirty, before unlocking the page which was supplied locked > by exofs_write_begin(). > > So I don't see where the page is PageDirty without being PageUptodate. > >> >> Now that page I do not want to read in. The latest data is in memory. >> (Same when this page is in writeback, dirty-bit is cleared) > > Understood, but that's what PageUptodate is for. > > (Quite what happens if there's a write error is not so clear: I think > that typically PageError gets set and PageUptodate cleared, to read > back in from disk what's actually there - but lose the data we wanted > to write; but I can understand different filesystems making different > choices there, and didn't study exofs's choice.) > >> >> So for sure if page is dirty or writeback then we surly do not need a read. >> only if not then we need to consider the PageUptodate(page) state. > > PageUptodate is the proper flag to check, to ask if the page contains > the correct data: there is no need to consider Dirty or Writeback. > >> >> Do you think the code is actually wrong as is? > > Not that I know of: just a little too complicated and confusing. > > But becomes slightly wrong if my simplification to page migration > goes through, since that introduces an instant when PageDirty is set > before the new page contains the correct data and is marked Uptodate. > Hence my patch. > >> >> BTW: Very similar code is in fs/nfs/objlayout/objio_osd.c::__r4w_get_page > > Indeed, the patch makes the same adjustment to that code too. > OK thanks. Let me setup and test your patch. On top of 4.3 is good? I'll send you a tested-by once I'm done. Boaz >> >>> Thanks, >>> Hugh >>> >> <> >> >> Thanks >> Boaz > -- 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>