Re: [Qemu-devel] [PATCH v7 08/35] exec: allow memory to be allocated from any kind of path

[Eduardo Habkost]

On Wed, Nov 04, 2015 at 11:12:41AM +0800, Xiao Guangrong wrote:
> On 11/04/2015 07:00 AM, Eduardo Habkost wrote:
> >On Mon, Nov 02, 2015 at 05:13:10PM +0800, Xiao Guangrong wrote:
> >>Currently file_ram_alloc() is designed for hugetlbfs, however, the memory
> >>of nvdimm can come from either raw pmem device eg, /dev/pmem, or the file
> >>locates at DAX enabled filesystem
> >>
> >>So this patch let it work on any kind of path
> >>
> >>Signed-off-by: Xiao Guangrong <address@hidden>
> >>---
> >>  exec.c | 24 ++++++++++++------------
> >>  1 file changed, 12 insertions(+), 12 deletions(-)
> >>
> >>diff --git a/exec.c b/exec.c
> >>index 9de38be..9075f4d 100644
> >>--- a/exec.c
> >>+++ b/exec.c
> >>@@ -1184,25 +1184,25 @@ static void *file_ram_alloc(RAMBlock *block,
> >>      char *c;
> >>      void *area;
> >>      int fd;
> >>-    uint64_t hpagesize;
> >>+    uint64_t pagesize;
> >>      Error *local_err = NULL;
> >>
> >>-    hpagesize = qemu_file_get_page_size(path, &local_err);
> >>+    pagesize = qemu_file_get_page_size(path, &local_err);
> >>      if (local_err) {
> >>          error_propagate(errp, local_err);
> >>          goto error;
> >>      }
> >>
> >>-    if (hpagesize == getpagesize()) {
> >>-        fprintf(stderr, "Warning: path not on HugeTLBFS: %s\n", path);
> >>+    if (pagesize == getpagesize()) {
> >>+        fprintf(stderr, "Memory is not allocated from HugeTlbfs.\n");
> >
> >If the point of this patch is to allow file_ram_alloc() to not be
> >specific to hugetlbfs anymore, this warning can simply go away.
> >
> >(And in case if you really want to keep the warning, I don't see the
> >point of the changes you made to it.)
> >
> 
> This is the history why we did it like this:
> https://lists.gnu.org/archive/html/qemu-devel/2015-10/msg02862.html

The rule I am trying to follow is simple: if there are valid use cases
(e.g. nvdimm, tmpfs) where the warning will be printed every single time
QEMU runs, the warning is not appropriate.

If you really want to keep a warning, please make it not be printed on
all other valid use cases (nvdimm and tmpfs). Personally, I don't think
adding those additional checks would be worth the trouble, that's why I
suggest removing the warning.

> 
> Q:
> | What this *actually* is trying to warn against is that
> | mapping a regular file (as opposed to hugetlbfs)
> | means transparent huge pages don't work.

I don't think the author of that warning even thought about transparent
huge pages (did THP even existed when it was written?). I believe they
just assumed that the only reason for using -mem-path would be hugetlbfs
and wanted to warn about it. That assumption isn't true anymore.

> 
> | So I don't think we should drop this warning completely.
> | Either let's add the nvdimm magic, or simply check the
> | page size.
> 
> A:
> | Check the page size sounds good, will check:
> | if (pagesize != getpagesize()) {
> |        ...print something...
> |}
> 
> | I agree with you that showing the info is needed, however,
> | 'Warning' might scare some users, how about drop this word or
> | just show　“Memory is not allocated from HugeTlbfs”?

With "warning:", I know it may be OK to do what I am doing and the
software is just trying to warn me. If there's no "warning:", I don't
even know if something is really broken in my config, or if it's just a
warning, and I would be very confused.

-- 
Eduardo