> From: 'David Cohen'
...
> > > The new kmalloc is going to be "n * sizeof(struct) - n * sizeof(pointer)"
> > > bigger. I don't know what is the usual range of values for "n", but my
> > > experience with android devices with non-abundant memory size is that
> > > they are sensible to kmalloc > PAGE_SIZE.
> >
> > It is much easier to assume that we are keeping the second malloc
> > are getting rid of the first one. The header is only one pointer.
>
> The header is not only one pointer. I believe the most relevant changes
> from your patch happen here:
My memory failed me - the 8 bytes are the two integers.
> --- a/drivers/usb/host/xhci.h
> +++ b/drivers/usb/host/xhci.h
> @@ -1363,7 +1363,7 @@ struct xhci_scratchpad {
> struct urb_priv {
> int length;
> int td_cnt;
> - struct xhci_td *td[0];
> + struct xhci_td td[0];
> };
>
> This is a dynamic array. It's not 1 pointer, it's how many you want it
> to be...
Yes, but each of the original td[] entries pointed to consecutive
entries of the original second malloc.
Maybe, at some point, they were each allocated separately.
> "sizeof(struct urb_priv)" does not consider the dynamic array at all, then
> you have the second value "size * sizeof(struct xhci_td *)" where "size"
> is the number of pointers you're going to have in the dynamic array.
> By doing your change you're increasing in the size of kmalloc in
> size * (sizeof(struct xhci_td) - sizeof(struct xhci_td *)) bytes.
Yes, but I'm removing a kmalloc of size * (sizeof (struct xhci_td)).
As I keep saying, I've moved the 'length' and 'td_cnt' fields to the
start of kmalloc that contains the actual data and completely removed
the allocation of an array of pointers (and all the code that dereferences
them).
sizeof (struct xhci_td) is something like 32, certainly less than 64 bytes.
So you need a moderate 'td_cnt' for the kmalloc to exceed a page size.
David
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