Make cacheRemoteRevalidating detect changes to file size

When file size changes outside the sandbox, page cache was not
refreshing file size which is required for cacheRemoteRevalidating.
In fact, cacheRemoteRevalidating should be skipping the cache
completely since it's not really benefiting from it. The cache is
cache is already bypassed for unstable attributes (see
cachePolicy.cacheUAttrs). And althought the cache is called to
map pages, they will always miss the cache and map directly from
the host.

Created a HostMappable struct that maps directly to the host and
use it for files with cacheRemoteRevalidating.

Closes #124

PiperOrigin-RevId: 230998440
Change-Id: Ic5f632eabe33b47241e05e98c95e9b2090ae08fc

1 files changed, 27 insertions, 3 deletions

diff --git a/pkg/sentry/fs/gofer/inode.go b/pkg/sentry/fs/gofer/inode.go
index f0dc99fd0..043705c58 100644
--- a/pkg/sentry/fs/gofer/inode.go
+++ b/pkg/sentry/fs/gofer/inode.go
@@ -125,6 +125,10 @@ type inodeFileState struct {
 	// failures. S/R is transparent to Sentry and the latter will continue
 	// using its cached values after restore.
 	savedUAttr *fs.UnstableAttr
+
+	// hostMappable is created when using 'cacheRemoteRevalidating' to map pages
+	// directly from host.
+	hostMappable *fsutil.HostMappable
 }
 
 // Release releases file handles.
@@ -166,6 +170,9 @@ func (i *inodeFileState) setHandlesForCachedIO(flags fs.FileFlags, h *handles) {
 			i.writebackRW = true
 		}
 	}
+	if i.hostMappable != nil {
+		i.hostMappable.UpdateFD(i.fdLocked())
+	}
 }
 
 // getCachedHandles returns any cached handles which would accelerate
@@ -287,7 +294,10 @@ func (i *inodeFileState) Sync(ctx context.Context) error {
 func (i *inodeFileState) FD() int {
 	i.handlesMu.RLock()
 	defer i.handlesMu.RUnlock()
+	return i.fdLocked()
+}
 
+func (i *inodeFileState) fdLocked() int {
 	// Assert that the file was actually opened.
 	if i.writeback == nil && i.readthrough == nil {
 		panic("cannot get host FD for a file that was never opened")
@@ -344,9 +354,13 @@ func (i *inodeOperations) Release(ctx context.Context) {
 
 // Mappable implements fs.InodeOperations.Mappable.
 func (i *inodeOperations) Mappable(inode *fs.Inode) memmap.Mappable {
-	if i.session().cachePolicy.usePageCache(inode) {
+	if i.session().cachePolicy.useCachingInodeOps(inode) {
 		return i.cachingInodeOps
 	}
+	// This check is necessary because it's returning an interface type.
+	if i.fileState.hostMappable != nil {
+		return i.fileState.hostMappable
+	}
 	return nil
 }
 
@@ -434,7 +448,7 @@ func (i *inodeOperations) NonBlockingOpen(ctx context.Context, p fs.PermMask) (*
 }
 
 func (i *inodeOperations) getFileDefault(ctx context.Context, d *fs.Dirent, flags fs.FileFlags) (*fs.File, error) {
-	if !i.session().cachePolicy.usePageCache(d.Inode) {
+	if !i.session().cachePolicy.cacheHandles(d.Inode) {
 		h, err := newHandles(ctx, i.fileState.file, flags)
 		if err != nil {
 			return nil, err
@@ -503,7 +517,7 @@ func (i *inodeOperations) SetTimestamps(ctx context.Context, inode *fs.Inode, ts
 // Truncate implements fs.InodeOperations.Truncate.
 func (i *inodeOperations) Truncate(ctx context.Context, inode *fs.Inode, length int64) error {
 	// This can only be called for files anyway.
-	if i.session().cachePolicy.usePageCache(inode) {
+	if i.session().cachePolicy.useCachingInodeOps(inode) {
 		return i.cachingInodeOps.Truncate(ctx, inode, length)
 	}
 
@@ -561,6 +575,16 @@ func (i *inodeOperations) StatFS(ctx context.Context) (fs.Info, error) {
 	return info, nil
 }
 
+func (i *inodeOperations) configureMMap(file *fs.File, opts *memmap.MMapOpts) error {
+	if i.session().cachePolicy.useCachingInodeOps(file.Dirent.Inode) {
+		return fsutil.GenericConfigureMMap(file, i.cachingInodeOps, opts)
+	}
+	if i.fileState.hostMappable != nil {
+		return fsutil.GenericConfigureMMap(file, i.fileState.hostMappable, opts)
+	}
+	return syserror.ENODEV
+}
+
 func init() {
 	syserror.AddErrorUnwrapper(func(err error) (syscall.Errno, bool) {
 		if _, ok := err.(p9.ErrSocket); ok {