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Wed, 22 Mar 2023 04:31:51 -0700 (PDT) Date: Wed, 22 Mar 2023 11:31:47 +0000 From: Lorenzo Stoakes To: David Hildenbrand Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Andrew Morton , Baoquan He , Uladzislau Rezki , Matthew Wilcox , Liu Shixin , Jiri Olsa , Jens Axboe , Alexander Viro Subject: Re: [PATCH v4 4/4] mm: vmalloc: convert vread() to vread_iter() Message-ID: References: <6b3899bbbf1f4bd6b7133c8b6f27b3a8791607b0.1679431886.git.lstoakes@gmail.com> <8ba0360e-57eb-93b0-3ae6-612f6b371bff@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8ba0360e-57eb-93b0-3ae6-612f6b371bff@redhat.com> X-Rspamd-Queue-Id: 4748180016 X-Rspamd-Server: rspam09 X-Rspam-User: X-Stat-Signature: 76dr68x7b9hdi5jcopwtw7beeru5weuk X-HE-Tag: 1679484714-953706 X-HE-Meta: U2FsdGVkX1/9nTZFucWzxEqyMGO8yavfdYOntCqGvZvwY4/kGLNwePubta8TvBzFFHENXArqu1MXrIO8zKrgA8vYAoQYXAOb6Vk/gjo9QXPBsI+JqC49AqJ0SorX5SNlO7pIqTsfmGD1ex4xGPtexUrh0m5SC6ApC6e8aW863TJQHDZ/xVmjU4t7xzTNDMvIdXDvb6tGQI4EGXg7sloPr3oc/3VxEtKH3NLC4NyEPjl5yp4CIpqGLtx9y299p8ANbz++yVQgILQt2qi7rfXAUz5NrC+/E8mrNTfODE04O2UzzT7CuY+ER4NiADEJASrG+iXWyhHL3DhlEZXz+u+siyTHM7Fvov7FD02l3wTQ3lMrPYZVupJjVsDfo0gNZsI765rk0PGrX1yfHNUPV5nghC5a/zo9XAgmJoGVKHqTBRvNO0FazfAljGYMPCxoOQjKJfevMT6EhNC8hDoeG/LeOuXyHNIk6+j0D2hBtLS51GO+9bwhYOaZLOuZ4lnvmIbi1H3160kZfAnm0Zbx7NpTwulvNuNMObBBpnOWiUk0/Hkbc2RpfiDqnLsXfKDnldnB1KKOPNBXWeYEne9Em1KUutBwtjE6kZsO5UffVYcYHaAQY66IGZagNjK9jTMUefAqKImWEAB+B/JPdN9wOKxTDimKoGhnsfZlg9eL+haduVbmiy6c/UGUG7AppQOL9xgiuW9OwuGsqimSD1gAi4IKuB05i4frkSy71C5aQOUXD6EJgerX+aF0vmHuHUP0TcShXXja20iXQZkCBJFhmxTYOOd4lildkk85eNMkSUFBezpa7Yt8RpSx/i/gk+CT+dlE1t7/2NgGSi9stnPvK+5oxgcA27QvkkKwA739oj0JyflPV8Gtjxve1TVyqw+UHJMuQePPrhj6YwwbdZBDy/X5CepEoSNlIlp4uPtngW7MDIihfaj+baK2Uk6YteVufUxoTs15bQ5s3Ar55c2/+Dn UELs0JC7 lejpZc9/nfh/TdVe2sL65/rybQ8UBvGYhPsflqIz46OXUtWw4SzSVldW195y4pqJKV5dsWeYGevlk5vCu2KL4Y0VntMv6He62Ql5BWcLFTCmybF5YD9nH8FGJE1CJ5JjSIu7GQQVDToAMBWMZQKR0I4pmwD1limJ2AK/FzjV3VMTkxC8omY4r155nVqEsdNjnQosaqQ1V04MVv84BeGJUNs+Uqpl74+JpxuBEMZ6tPOoGUlBVh12hiAy29rlVT2SwNEI+NrLTflXCleRau/B8kpK8iMRjVaDXwFTpR9dthmtJjyl8QXtEoZn2K+BQDmmQ/S5FJsicGcB9ByRVmykyCLkZLmWxV70wpG3b9QxmqxynCFNy6VKu4sY7YbEs/6ruyVreUzemQ3RZYOTGfLeAlrJT8i/VFj7SgyCd5rRvDCjrZ2dUSWDE29vQqtHYIf27st8aTq3ncstgvMoVkehflKmtR59M+EG7B19BP5nhB86m8qo= 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 Wed, Mar 22, 2023 at 12:18:08PM +0100, David Hildenbrand wrote: > On 21.03.23 21:54, Lorenzo Stoakes wrote: > > Having previously laid the foundation for converting vread() to an iterator > > function, pull the trigger and do so. > > > > This patch attempts to provide minimal refactoring and to reflect the > > existing logic as best we can, for example we continue to zero portions of > > memory not read, as before. > > > > Overall, there should be no functional difference other than a performance > > improvement in /proc/kcore access to vmalloc regions. > > > > Now we have eliminated the need for a bounce buffer in read_kcore_iter(), > > we dispense with it. We need to ensure userland pages are faulted in before > > proceeding, as we take spin locks. > > > > Additionally, we must account for the fact that at any point a copy may > > fail if this happens, we exit indicating fewer bytes retrieved than > > expected. > > > > Signed-off-by: Lorenzo Stoakes > > --- > > fs/proc/kcore.c | 26 ++--- > > include/linux/vmalloc.h | 3 +- > > mm/nommu.c | 10 +- > > mm/vmalloc.c | 234 +++++++++++++++++++++++++--------------- > > 4 files changed, 160 insertions(+), 113 deletions(-) > > > > diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c > > index 25e0eeb8d498..221e16f75ba5 100644 > > --- a/fs/proc/kcore.c > > +++ b/fs/proc/kcore.c > > @@ -307,13 +307,9 @@ static void append_kcore_note(char *notes, size_t *i, const char *name, > > *i = ALIGN(*i + descsz, 4); > > } > > -static ssize_t > > -read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter) > > +static ssize_t read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter) > > { > > - struct file *file = iocb->ki_filp; > > - char *buf = file->private_data; > > loff_t *ppos = &iocb->ki_pos; > > - > > size_t phdrs_offset, notes_offset, data_offset; > > size_t page_offline_frozen = 1; > > size_t phdrs_len, notes_len; > > @@ -507,9 +503,12 @@ read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter) > > switch (m->type) { > > case KCORE_VMALLOC: > > - vread(buf, (char *)start, tsz); > > - /* we have to zero-fill user buffer even if no read */ > > - if (copy_to_iter(buf, tsz, iter) != tsz) { > > + /* > > + * Make sure user pages are faulted in as we acquire > > + * spinlocks in vread_iter(). > > + */ > > + if (fault_in_iov_iter_writeable(iter, tsz) || > > + vread_iter(iter, (char *)start, tsz) != tsz) { > > ret = -EFAULT; > > goto out; > > } > > What if we race with swapout after faulting the pages in? Or some other > mechanism to write-protect the user space pages? > > Also, "This is primarily useful when we already know that some or all of the > pages in @i aren't in memory". This order of events might slow down things > quite a bit if I am not wrong. > > > Wouldn't you want to have something like: > > while (vread_iter(iter, (char *)start, tsz) != tsz) { > if (fault_in_iov_iter_writeable(iter, tsz)) { > ret = -EFAULT; > goto out; > } > } > > Or am I missing something? > Indeed, I was thinking of this as:- - prefault - try (possibly fail if race) copy operation However it does make more sense, and makes it explicit that it's an attempt that might fail requiring a fault-in in the while form. I think the upcoming change to explicitly make the iter function copy_folio_to_iter_nofault() makes it clear from end-to-end what is being done - we mustn't fault (spinlocks held) and if a fault would occur we error out, if we error out try faulting in, if this fails then abort. I will fixup in respin. > -- > Thanks, > > David / dhildenb >