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Sat, 18 Jan 2020 21:55:05 -0800 (PST) MIME-Version: 1.0 References: <20200117074534.25324-1-richardw.yang@linux.intel.com> <20200117222740.GB29229@richard> <20200117234829.GA2844@richard> <20200119024124.GF9745@richard> In-Reply-To: <20200119024124.GF9745@richard> From: Yang Shi Date: Sat, 18 Jan 2020 21:54:50 -0800 Message-ID: Subject: Re: [PATCH] mm/migrate.c: also overwrite error when it is bigger than zero To: Wei Yang Cc: Andrew Morton , Linux MM , Linux Kernel Mailing List Content-Type: text/plain; charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Sat, Jan 18, 2020 at 6:41 PM Wei Yang wrote: > > On Fri, Jan 17, 2020 at 08:56:27PM -0800, Yang Shi wrote: > >On Fri, Jan 17, 2020 at 3:48 PM Wei Yang wrote: > >> > >> On Fri, Jan 17, 2020 at 03:30:18PM -0800, Yang Shi wrote: > >> >On Fri, Jan 17, 2020 at 2:27 PM Wei Yang wrote: > >> >> > >> >> On Fri, Jan 17, 2020 at 03:45:34PM +0800, Wei Yang wrote: > >> >> >If we get here after successfully adding page to list, err would be > >> >> >the number of pages in the list. > >> >> > > >> >> >Current code has two problems: > >> >> > > >> >> > * on success, 0 is not returned > >> >> > * on error, the real error code is not returned > >> >> > > >> >> > >> >> Well, this breaks the user interface. User would receive 1 even the migration > >> >> succeed. > >> >> > >> >> The change is introduced by e0153fc2c760 ("mm: move_pages: return valid node > >> >> id in status if the page is already on the target node"). > >> > > >> >Yes, it may return a value which is > 0. But, it seems do_pages_move() > >> >could return > 0 value even before this commit. > >> > > >> >For example, if I read the code correctly, it would do: > >> > > >> >If we already have some pages on the queue then > >> >add_page_for_migration() return error, then do_move_pages_to_node() is > >> >called, but it may return > 0 value (the number of pages that were > >> >*not* migrated by migrate_pages()), then the code flow would just jump > >> >to "out" and return the value. And, it may happen to be 1. > >> > > >> > >> This is another point I think current code is not working well. And actually, > >> the behavior is not well defined or our kernel is broken for a while. > > > >Yes, we already spotted a few mismatches, inconsistencies and edge > >cases in these NUMA APIs. > > > >> > >> When you look at the man page, it says: > >> > >> RETURN VALUE > >> On success move_pages() returns zero. On error, it returns -1, and sets errno to indicate the error > >> > >> So per my understanding, the design is to return -1 on error instead of the > >> pages not managed to move. > > > >So do I. > > > >> > >> For the user interface, if original code check 0 for success, your change > >> breaks it. Because your code would return 1 instead of 0. Suppose most user > >> just read the man page for programming instead of reading the kernel source > >> code. I believe we need to fix it. > > > >Yes, I definitely agree we need fix it. But the commit log looks > >confusing, particularly "on error, the real error code is not > >returned". If the error is returned by add_page_for_migration() then > >it will not be returned to userspace instead of reporting via status. > >Do you mean this? > > > > Sorry for the confusion. > > Here I mean, if add_page_for_migratioin() return 1, and the following err1 > from do_move_pages_to_node() is set, the err1 is not returned. > > The reason is err is not 0 at this point. Yes, I see your point. Please elaborate this in the commit log. > > -- > Wei Yang > Help you, Help me