On Thu, Mar 23, 2023 at 12:50 PM Chris Li <chrisl@xxxxxxxxxx> wrote: > > On Thu, Mar 23, 2023 at 11:40:35AM -0700, Yosry Ahmed wrote: > > > Thanks for the clarification. If we have swap_desc index in the PTE, > > > let's call it S1. I assume S1 will contain A1 and B1 as part of the > > > swap_desc struct. > > > > > > Now we are getting to some very interesting details. > > > > > > What is the life cycle of the S1? Does S1 share the same index as A1? > > > > The idea that we are currently discussing does not involve a swap_desc > > struct. There is only a swap ID that indexes into an xarray that > > points to a swap_entry. This swap ID and the xarray formulate the > > indirection layer. > > I see. The swap_entry is the same size as the swp_entry_t, which is > unsigned long. > > > I am guessing in this design the swap ID is allocated when unmapping a > > page to be swapped out, and freed when the underlying swap_entry's > > swap count falls to 0. > > If none of the swap pages on A need to move to B (yet). Does it still > need to allocate an entry in the swap ID xarray, only pointing to A1? > > At the time of swap out page into A, we will not know if it will > move to B in a later time. I guess the swap ID xarray look up always > needs to be there? If the indirection is enabled, yes. > > > Moving a page from a swap backend A to another swap backend B should > > not be a problem in terms of the swap cache, as we will add it to the > > swap cache of B, modify the swap ID mapping to point to B, then remove > > it from the swap cache of A. > > That means when B swap in a page, it will always look up the swap ID > xarray first, then resolve to the actual swap_entry B1. Yes. There is an extra lookup. > > > There are some concerns with this design that I outlined in one of my > > previous emails, such as having separate swap counting implementation > > in different swap backends, which is a maintenance burden and > > error-prone. > > I agree that allocating the swap ID and maintaining the free swap ID > would be some extra complexity if we are not reusing the existing swap > count code path. > > My other concern would be the swap ID xarray indirection is always there > regardless if you need to use the indirection or not. I think the idea is that this design is more minimal than the proposed swap_desc, so we can have it behind a config option and remove the indirection layer if it is not configured. However, I am not yet sure if this would be straightforward. I need to give this more thought. > > Chris
