Messages in this thread | | | Date | Wed, 22 Mar 2023 12:18:08 +0100 | Subject | Re: [PATCH v4 4/4] mm: vmalloc: convert vread() to vread_iter() | From | David Hildenbrand <> |
| |
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 <lstoakes@gmail.com> > --- > 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?
-- Thanks,
David / dhildenb
| |